mediatek: add a new spi-nand driver for kernel 5.10

mediatek: add a new spi-nand driver for kernel 5.10

This patch adds a new spi-nand driver which implements the SNFI of mt7622
and mt7629.

Unlike the existing snfi driver which makes use of the spi-mem framework
and the spi-nand framework with modified ecc support, this driver is
implemented directly on the mtd framework with other components untouched,
and provides better performance, and behaves exactly the same as the nand
framework.

Signed-off-by: Weijie Gao <hackpascal@gmail.com>