sf: ops: Add spi_flash_copy_mmap function

When doing a memory mapped copy we may have DMA available and thus need
to have this copy abstracted so that the driver can do it, rather than a
simple memcpy.

Signed-off-by: Tom Rini <trini@ti.com>
Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>