mediatek: add support for the new MT7623 Arm SoC

the support is still WIP. next steps are to make the pmic and ethernet work.
this is the first commit to make sure nothing gets lost.

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47354 3c298f89-4303-0410-b956-a3cf2f4a3e73