nrf52840_m2 add backlight configuration and pinctrl

[BarbUk]

Hi !

Following the dev page available at:

• https://wiki.makerdiary.com/m60/developer_guide/hardware/#leds-on-m2
• https://wiki.makerdiary.com/nrf52840-m2/resources/nrf52840_m2_module_diagram_v1_0.pdf

I've added the necessary pinctrl for the nrf52840_m2 board.

pwm configuration come from:

Spi config from:

I've also added the i2c pinctrl to add a mapping with the is31fl3733 driver in a futur PR.

PR check-list

• [X] Branch has a clean commit history
• [X] Additional tests are included, if changing behaviors/core code that is testable.
• [X] Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• [X] Pre-commit used to check formatting of files, commit messages, etc.
• [X] Includes any necessary documentation changes.