regulator: spm-regulator: fall back on SPMI write if msm_spm_set_vdd fails

Update the spm-regulator driver so that it falls back on an SPMI
register write if the msm_spm_set_vdd() call to set the voltage
of a regulator fails.  This retry mechanism is needed because
msm_spm_set_vdd() is not guaranteed to work for all regulators at
all times.  It can only work if at least one processor core
powered by the regulator is online.  Unfortunately, the clock
driver may request a voltage during the process of bringing the
first such core online.

Change-Id: I93980d603a324da2b8d21e0b78958ff5032168b0
Signed-off-by: David Collins <collinsd@codeaurora.org>