spi: orion: Add multiple chip select support to spi-orion

This commit adds support for multiple hardware chip selects to spi-orion.
Different SoCs support different number of chip selects (up to
8 on some platforms). The driver allows up to this number, and it is up
to the implementer to only use the chip selects that are available.

Signed-off-by: Ken Wilson <ken.wilson@opengear.com>
Signed-off-by: Mark Brown <broonie@kernel.org>