sf: Add memory mapped read support

Qspi controller can have a memory mapped port which can be used for
data read. Added support to enable memory mapped port read.

This patch enables the following:
- It enables exchange of memory map address between mtd and qspi
through the introduction of "memory_map" flag.
- Add support to communicate to the driver that memory mapped
 transfer is to be started through introduction of new flags like
"SPI_XFER_MEM_MAP" and "SPI_XFER_MEM_MAP_END".

This will enable the spi controller to do memory mapped configurations
if required.

Signed-off-by: Sourav Poddar <sourav.poddar@ti.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>