pinctrl: get rid of some ifdeffery

Don't define an empty version for pinconfig_post_bind(). Just guard the
call and let the linker garbage collection do the rest. This way, we
also don't have to do any guesswork.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Marek Vasut <marex@denx.de>