Initial import of Ralink/Mediatek MIPS SoC support #7

GPIO controller drivers import.

As with other Ralink/Mediatek work, there are 2 versions of the GPIO
controller driver, depending on the type of SoC.
This revision introduces initial support for these.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5877