spi: orion: Handle GPIO chip-selects

Some hardware designs use GPIOs to add (or supplement) the SPI
chip-select so that more than one SPI slave device can be used.

For this to work with the spi-orion driver the SPI_MASTER_GPIO_SS flag
needs to be set (because the other outputs are gated internally by the
CS) and the correct chip-select (in this case CS0) needs to be driven by
the controller.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Mark Brown <broonie@kernel.org>