mtd: spi: spi-nor-core: Add SPI MEM support

Many SPI controllers have special MMIO interfaces which provide
accelerated read/write access but require knowledge of flash parameters
to make use of it. Recent spi-mem layer provides a way to support such
controllers.
Therefore, add spi-mem support to spi-nor-core as a way to support SPI
controllers with MMIO interface. SPI MEM layer takes care of translating
spi_mem_ops to spi_xfer()s in case of legacy SPI controllers.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Tested-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Tested-by: Stefan Roese <sr@denx.de>
Tested-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> #zynq-microzed