regulator: cpr-regulator: add support for per-RO CPR step quotient

Add support for using a different CPR step quotient value for
each possible ring oscillator (RO).  This ensures that equivalent
CPR dynamic voltage margins can be achieved on a given chip
regardless of which RO is used.

Also clear all RBCPR_GCNT_TARGET registers before switching
voltage corners so that only the expected RO is used.  This is
needed in order to support different RO's for each fuse corner.

CRs-Fixed: 762377
Change-Id: If28d30c0f979669f46936f4e064119a69834d1e1
Signed-off-by: David Collins <collinsd@codeaurora.org>