ARM: dts: sunxi: Add regulators for LeMaker BananaPi

sun7i-a20-bananapi.dts doesn't contain regulator nodes for the AXP209 PMU
driver, so add them to allow for voltage-scaling with cpufreq-dt. Also
add board-specific OPP to use slightly higher voltages at lower
frequencies since Kevin Hilman reported that not all BananaPi boards run
stable at the default voltages inherited by sun7i-a20.dtsi.

Signed-off-by: Timo Sigurdsson <public_timo.s@silentcreek.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>