pxa2xx_spi: fix for SPI_CS_HIGH

Commit a7bb3909b3293d503211d7f6af8ed62c1644b686 ("spi: pxa2xx_spi:
introduce chipselect GPIO to simplify the common cases") introduces
chipselect GPIO, and configures the CS polarity using SPI_CS_HIGH
spi->mode flag.  Add SPI_CS_HIGH to the allowed modes.

Signed-off-by: Daniel Ribeiro <drwyrm@gmail.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>