i2c-mux-gpio: Update documentation
i2c-mux-gpio: Update documentation

* Document the possibility to pass relative GPIO pin numbers.
* Document what platform device IDs to use, so that they do not
  collide.

Signed-off-by: Jean Delvare <49ad6a9f5aa17024c23048df346d55bda6837e01@linux-fr.org>
Cc: Peter Korsgaard <6b4d40de4b806e606a54c5b4518dbd05b106c43d@barco.com>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas