[MIPS] rbtx4938: Add generic GPIO support

GPIO 0..15 are for TX4938 PIO pins, GPIO 16..18 are for FPGA-driven
chipselect signals for SPI devices.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
3 files changed