Initial import of Ralink/Mediatek MIPS SoC support #6

SPI drivers for the various Ralink/Mediatek SoCs. There are 2 versions of
the SPI controller (so far) present in the supported SoCs, hence v1 and v2
drivers.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5842