spi: spi-mem: export spi_mem_default_supports_op()

Controllers can use this function to perform basic sanity checking on
the spi-mem op.

Reviewed-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>