[mips] Pin control configuration for MediaTek RT2880
RT2880 is MIPS4Kc SoC used in many SOHO routers. This commits adds GPIO pin
control configuration of RT2880.
Submitted by: Hiroki Mori <yamori83@yahoo.co.jp>
Reviewed by: mizhka, sgalabov
Differential Revision: https://reviews.freebsd.org/D12648