spi: Add QuadSPI driver for Atmel SAMA5D2

Kernel contains QSPI driver strongly tied to MTD and nor-flash memory.
New spi-mem interface allows usage also other memory types, especially
much larger NAND with SPI interface. This driver works as SPI controller
and is not related to MTD, however can work with NAND-flash or other
peripherals using spi-mem interface.

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>