spi: add spi controller support for MediaTek MT7620 SoC

This patch adds spi controller support for MediaTek MT7620 SoC.

The SPI controller supports two chip selects. These two chip selects are
implemented as two separate register groups, but they share the same bus
(DI/DO/CLK), only CS pins are dedicated for each register group.
Appearently these two register groups cannot operates simulataneously so
they are implemented as one controller.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>