mtd: spi-nor: atmel-quadspi: Use spi-mem interface for atmel-quadspi driver

Previously added spi-mem interface is now used instead of older approach.

Suggested-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Piotr Bugalski <bugalski.piotr@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>