MIPS: RB532: Remove gpio bootup state

We are no longer using gpio bootup state, so do not export
it and do not parse the kernel command line tag for it.
Instead we provide gpio-keys for the button the gpio bootup
state was checking.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>