Selects GENERIC_GPIO for ARCH_IXP4XX in Kconfig.

IXP4XX has generic GPIO support; however, ARCH_IXP4XX
Kconfig entry currently does not select GENERIC_GPIO like other
arch entries.
Signed-off-by: Milan Svoboda <msvoboda@ra.rockwell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>