mtd: spi-nor: mtk-quadspi: rename config to a common one

The quadspi is a generic communication interface which could be shared
with other MediaTek SoCs. Hence rename it to a common one.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>