spi: bcm-qspi: Implement the spi_mem interface

The spi_mem interface is meant to replace the ->spi_flash_read() one.
Implement the ->exec_op() method to ease removal of the old interface.

Not that ->spi_flash_read() is now implemented as a wrapper around the
new bcm_qspi_exec_mem_op() function so that we can easily get rid of
it when ->spi_flash_read() is removed.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Kamal Dasu <kdasu.kdev@gmail.com>
Tested-by: Kamal Dasu <kdasu.kdev@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>