bcm53xx: add support for Asus RT-N18U LEDs & buttons

Got GPIOs from #18152 thanks to Grrruk

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43010 3c298f89-4303-0410-b956-a3cf2f4a3e73