Commit 5fa5e6de ("spi: atmel: Switch to transfer_one transfer
method") switched to using transfer_one and set_cs.  The
core doesn't call set_cs when the chip select lines are gpios.  Add the
SPI_MASTER_GPIO_SS flag to the driver to ensure the calls to set_cs
happen since the driver programs configuration registers there.

Fixes: 5fa5e6de ("spi: atmel: Switch to transfer_one transfer method")
Signed-off-by: Dan Sneddon <dan.sneddon@microchip.com>
Link: https://lore.kernel.org/r/20210629192218.32125-1-dan.sneddon@microchip.comSigned-off-by: Mark Brown <broonie@kernel.org>