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>
diff --git a/drivers/spi/Kconfig b/drivers/spi/Kconfig
index cd19b2d..1efb5b1 100644
--- a/drivers/spi/Kconfig
+++ b/drivers/spi/Kconfig
@@ -210,6 +210,13 @@
 	  Enable MSCC bitbang SPI driver. This driver can be used on
 	  MSCC SOCs.
 
+config MT7620_SPI
+	bool "MediaTek MT7620 SPI driver"
+	depends on SOC_MT7620
+	help
+	  Enable the MT7620 SPI driver. This driver can be used to access
+	  generic SPI devices on MediaTek MT7620 SoC.
+
 config MT7621_SPI
 	bool "MediaTek MT7621 SPI driver"
 	depends on SOC_MT7628