mtd: spi-nor: atmel-quadspi: Add spi-mem support to atmel-quadspi

This patch adds new interface to existing driver. New code is not used yet,
it will be enabled later.
Changes are prepared in small steps to keep patches readable.

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>