spi: pxa2xx: Use LPSS prefix for defines that are Intel LPSS specific

Rename a few defines that are specific to Intel LPSS SPI private
registers with LPSS prefix. It makes easier to distinguish them from
common defines.

Suggested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>