ARM: dts: sunxi: unify APB1 clock

With the new factors infrastructure in place, we can unify apb1 and
apb1_mux as a single clock now.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
[wens@csie.org: Change apb1 node label to "apb1"; reword commit title]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>