regulator: cpr3-regulator: add support for system supply voltage requests

Add support for setting the voltage of the system-supply
regulator when the voltage of the vdd-supply regulator is scaled.
The vdd-supply regulator may optionally depend upon the
system-supply regulator.  For example, the system-supply could be
configured as the VDD_CX or VDD_MX supply in order to satisfy
hardware requirements.

Change-Id: I652ddc9cf80d531de49d6fa5028d8974d65bc51c
Signed-off-by: David Collins <collinsd@codeaurora.org>