msm: spm: Replace msm_spm_apcs_set_vdd with msm_spm_set_vdd

msm_spm_apcs_set_vdd() is used to set the vdd of a gang rail through the
L2 SAW2. Remove this api and overload the msm_spm_set_vdd() call with a
dont-care 'cpu' argument instead.

Detect if the target is configured to use L2 SAW as APCS master using a
boolean property in the device tree.

Change-Id: Ib8d9605474fa44d5223985ec8cb1040cde230592
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>