power: qpnp-fg-gen3: add DMA support for accessing FG SRAM

Currently FG SRAM is accessed by Interleaved Memory Access mode
(IMA). GEN3 FG has support for Direct Memory Access (DMA) to
access the SRAM. With DMA, SRAM can be directly accessed via the
SPMI peripheral registers once the configuration is done and DMA
is granted. This improves the latency in accessing SRAM with IMA.

Change-Id: I6208e75b23a07e983c4aee103454076ac66b9806
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>