arm64: rockchip: gpio: Give friendlier name to gpio

By default name the gpio P<bank><bankpin>
This make it easier to find the gpio when reading schematics or DTS.

Sponsored by: Diablotin Systems
Differential Revision: https://reviews.freebsd.org/D30287

(cherry picked from commit bc1bb80564eef66cef1a7b9791af87ae019aa71d)