ARM: sa1100/assabet: add BCR/BSR GPIO driver

Add a GPIO driver for the board control register/board status register
for the sa1100/assabet platform.  This allows us to transition a range
of drivers to use the gpiod APIs rather than the platform private
ASSABET_BCR_* interfaces.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>