spi: pxa2xx_spi: introduce chipselect GPIO to simplify the common cases

Most SPI peripherals use GPIOs as their chip selects, introduce .gpio_cs
for this.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Cc: David Brownell <david-b@pacbell.net>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>