Add PSRAM support

[will-v-pi]

This adds a hardware_psram library to the SDK to add PSRAM support, as requested in #2372

Adds PSRAM defines to board headers for boards that have PSRAM - I may not have captured them all

Has three ways it will detect PSRAM when you link hardware_psram, in order of priority:

1. If flash_devinfo is setup (eg configured in OTP), it will initialise PSRAM with the flash_devinfo CS1 size and GPIO
2. If PICO_AUTO_DETECT_PSRAM is set it will attempt to detect PSRAM size and GPIO on CS1
3. If PICO_PSRAM_SIZE_BYTES is set (eg configured in board header, or with pico_override_psram_size) it will initialise PSRAM with that size and GPIO

Variables can be placed in PSRAM using __psram or __psram_uninitialised macros, and you can also write directly to the memory addresses.

If there are variables placed in PSRAM, it will setup ATRANS to prevent any access to PSRAM addresses greater than the size available (from flash_devinfo, auto-detection, or PICO_PSRAM_SIZE_BYTES), which will cause bus faults. The psram_is_available function should be used before accessing variables in PSRAM when auto-detection is on, to prevent these bus faults.

This support uses flash_devinfo to enable the bootrom support for PSRAM, then adds a callback with flash_set_qmi_cs1_setup_function which runs as part of flash_rp2350_restore_qmi_cs1 to enable quad mode and setup faster timings. The RP2350-E14 workaround is included.

The CS pin auto-detection iterates through all possible CS pins, skipping any that are defined as defaults in board header files with remove_defaults_from_cs_gpios - this is an ugly function, but without it you get some garbled characters on the UART as pin 0 is a possible CS pin. Maybe some of the defaults could be removed from this, if they are unlikely to be affected by this auto-detection?

This has psram_or_malloc and psram_or_free macros, which statically allocate in PSRAM and use that if available, but use malloc if not available - not sure how useful these are, could be removed if we don't want them. They were to demostrate psram_check_address which checks if an address is in available PSRAM, so you can avoid busfaults when using variables in PSRAM.

In passing this also:

• Removes some Bazel TODOs - CMake gates RP2350 only stuff in src/cmake/rp2_common.cmake
• Expands RP2350 boot2 linker script by 4 bytes, as there is no checksum

[lurch]

Hmm, I wonder if there's a use-case for being able to say "never attempt to probe this list-of-GPIOs for PSRAM CS" if there are certain pins which the user doesn't want to get wiggled, but which aren't covered by any of the board-header _DEFAULT_ defines? 🤔

[will-v-pi]

Maybe, and that could be a better way of including the default pins

[lurch]

Should this PR also be adding hardware_psram to docs/index.h ?
Should line 15 of src/rp2_common/hardware_psram/include/hardware/psram.h be declaring hardware_psram (as used on e.g. line 133) instead of pico_psram?