sh: Fix up GENERIC_GPIO build for ARCH_WANT_OPTIONAL_GPIO cases.

CPUs define pinmux tables through the optional interface, while boards
that require demux of their own require it explicitly. Roll the Makefile
rules back to depend on GENERIC_GPIO, which covers both cases.

Fixes a link error with an undefined reference to register_pinmux() on
optional platforms.

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