spi: pxa2xx: Detect number of enabled Intel LPSS SPI chip select signals

SPI capabilities register located in private registers space of newer
Intel LPSS SPI host controllers tell in register bits 12:9 which chip
select signals are enabled.

Use that information for detecting the number of chip selects. For
simplicity we assume chip selects are enabled one after another without
disabled chip selects between. For instance CS0 | CS1 | CS2 but not
CS0 | CS1 | CS3.

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