sh: Only build in gpio.o when CONFIG_GENERIC_GPIO is selected.

The pinmux management is tied in to this code, while it is presently only
used by platforms that select GENERIC_GPIO. The asm/gpio.h definitions
are not referenced when GENERIC_GPIO is disabled, resulting in a build
failure for all of the platforms that don't select it.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>