[mediatek] Add support for non-flash devices on the SPI bus of the Mediatek SoCs.

The existing SPI support only worked for directly attached flash chips.
it didn't implement clock programming or chipselect. It also supports
transfers with unbalanced tx/rx command sizes.

Submitted by: <yamori813@yahoo.co.jp>
Differential Revision: https://reviews.freebsd.org/D20101