MIPS: JZ4770: Added minimal JZ4740-like gpio.h header

I only included the things needed to break the dependency on
jz4770gpio.h, which contains the register definitions as well.

Most of the omitted definitions will be obsoleted anyway when pinctrl
is adopted for the JZ4740.
ASoC: JZ4770: GCW0: Use new gpio header
MIPS: JZ4770: Removed /proc/jz