spi: pxa2xx: Add support for Intel Broxton

LPSS SPI in Intel Broxton is otherwise the same than in Intel Sunrisepoint
but it supports up to four chip selects per port and has different FIFO
thresholds. Patch adds support for two Broxton SoC variants.

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