spi: pxa2xx: Add output control for multiple Intel LPSS chip selects

Intel LPSS SPI host controllers in upcoming Intel platforms can have up
to 4 chip selects per port. Extend chip select control in
lpss_ssp_cs_control() by adding a code that selects the active chip
select output prior to changing the state. Detection for number of
enabled chip select signals will be added by another patch.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>