spi: mtk_snor: add support for MTK SPI NOR controller

This patch adds support for MTK SPI NOR controller, which you
can see on mt7622 & mt7629.

1. This controller is designed only for SPI NOR. We can't adjust
its bus clock dynamically. Set clock in dts instead.
2. This controller only supports 1-1-1 write mode.
3. Remove mtk_snor_match_read() since upper SPI-MEM layer already
handles command.
4. sf read/write/update commands are tested with this driver.

diff --git a/drivers/spi/Kconfig b/drivers/spi/Kconfig
index 1efb5b1..1494c91 100644
--- a/drivers/spi/Kconfig
+++ b/drivers/spi/Kconfig
@@ -225,6 +225,13 @@
 	  the SPI NOR flash on platforms embedding this Ralink / MediaTek
 	  SPI core, like MT7621/7628/7688.
 
+config MTK_SNOR
+	bool "Mediatek SPI-NOR controller driver"
+	depends on SPI_MEM
+	help
+	  Enable the Mediatek SPINOR controller driver. This driver has
+          better read/write performance with NOR.
+
 config MTK_SNFI_SPI
 	bool "Mediatek SPI memory controller driver"
 	depends on SPI_MEM