ramips: add support for PandoraBox PBR-M1

This patch is for PandoraBox PBR-M1 which is based on mt7621,
all the features work fine, including rtc, leds, button, usb3.0, etc.

Signed-off-by: tymon <banglang.huang@foxmail.com>

Backport of r46041

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46051 3c298f89-4303-0410-b956-a3cf2f4a3e73