MIPS: BCM47XX: Define Linksys WRT310N V2 buttons

Update the buttons registration code to register the two buttons (WPS,
system rester) using the existing BCM47XX_BOARD_LINKSYS_WRT310NV2 board
entry.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>