boards/nrf5340dk-app: Map out saul_gpio

[chrysn]

Contribution description

All the Nordic DKs have 4 buttons and 4 LEDs, but for the nRF5340 they were not mapped out.

One could set up PWMs to make the LEDs dimmable, but let's not have the better be the enemy of the good, and also, I don't know how to translate Fleißaufgabe.

Testing procedure

Running the SAUL example:

> saul
saul
ID      Class           Name
#0      SENSE_BTN       Button 1
#1      SENSE_BTN       Button 2
#2      SENSE_BTN       Button 2
#3      SENSE_BTN       Button 4
#4      ACT_SWITCH      LED1
#5      ACT_SWITCH      LED2
#6      ACT_SWITCH      LED3
#7      ACT_SWITCH      LED4
> saul read 0
saul read 0
Reading from #0 (Button 1|SENSE_BTN)
Data:                 0 
> saul write 4 1
saul write 4 1
Writing to device #4 - LED1
Data:                 1 
data successfully written to device #4

(And to be thorough, I wrote to all the LEDs in sequence to check their maping, and did the read-all for every single pressed button.)

Issues/PRs references

Based on #22289

Declaration of AI-Tools / LLMs usage:

AI-Tools / LLMs that were used are:

• none

[riot-ci]

Murdock results

✔️ PASSED

ed494ca boards/nrf5340dk-app: Map buttons and LEDs through SAUL

Success	Failures	Total	Runtime
565	0	565	04m:54s

Artifacts

• Documentation preview

[crasbe]

This doesn't have to be part of this PR, does it?

Because if it doesn't, I'd trust your testing and merge the changes already before the situation in #22289 is resolved 🤔 🤷‍♂️

[chrysn]

Indeed, sorry -- it only mattered for my extra tests.

I'm taking the liberty to rebase, as things are just messy otherwise.