arm: dts: change MT7629 to use spi-mem rather than qspi

The original mtk_qspi driver has been removed. We change MT7629 to use
newly added mtk-spimem driver.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>