spi: mtk_snor: add support for MTK SPI NOR controller

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.

Signed-off-by: SkyLake.Huang <skylake.huang@mediatek.com>