power: fg-memif: Add IMA support to access SRAM on GEN4 FG

Though DMA is the primary method for accessing FG SRAM, having
IMA (Interleaved memory access) support will be handy. Since the
address range supported is almost twice and each word address
represents 2 bytes, make the changes to support it for GEN4 FG.

Change-Id: Ib21a91f865ef0191f7100ed5084aa7aef137b52f
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>