msm: spm-v2: Add API to enable PFM mode on PMIC

When the Kraits are drawing the least power, the Krait buck can be put
in low power mode (PFM) as well.
The msm_spm_enable_fts_lpm() API will allow the PFM mode to be enabled
through the SPM PMIC interface.

Change-Id: Ia5fd16e27e2c90683f6ae93b1e909fa79722a091
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>