MIPS: BCM47XX: Import buttons database from OpenWrt

This includes all devices from OpenWrt's "diag" that we support in arch
code (we have entries for in enum bcm47xx_board).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6301/