MAINTAINERS: add myself as a collaborator in a few areas

Added myself as a collaborator to the areas I frequently contribute or
review code. List includes:

- Display drivers
- Documentation
- Kscan drivers
- Sensor drivers
- Power management
- STM32 platform
- Pinmux/pinctrl drivers
- PWM drivers

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>