spi: pxa2xx: Add support for Intel Broxton B-Step

Add support for third Intel Broxton variant and update comment for
A-Step variant.

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