dt-bindings: regulator: rpmh-regulator: add new intermediate voltage levels

Add constants for voltage levels: LOW_SVS_L1, LOW_SVS_L2,
and SVS_L0.

Change-Id: I5dc38090f37ceb83b32fade10830e873ebeb3610
Signed-off-by: David Collins <collinsd@codeaurora.org>