ARM: sunxi: dt: add nodes for the mbus clock

mbus is the memory bus clock, and it is present on both sun5i and sun7i
machines. Its register layout is compatible with the mod0 one.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>