allwinner: rsb: Set the PMIC to RSB mode

With recent ATF (v2.5) the PMIC is reset to I2C mode.
Without a PMIC no regulators can be changed/enabled/disabled
This fixes cpufreq on A64 (at least) and anything else that needs
regulators handled by the PMIC.

MFC after: 3 days
Sponsored by: Diablotin Systems