spi: mtk_qspi: add qspi driver for MT7629 SoC

This patch adds MT7629 qspi driver for accessing SPI NOR flash.

Signed-off-by: Guochun Mao <guochun.mao@mediatek.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>