power: qpnp-fg: add SRAM debugfs

This adds the fg_memif directory in debugfs with
four entries:

  - address: specify a 12 bit address within the SRAM
  - data: a user can specify a decimal or hexadecimal
          string to write to the SRAM with the length
          of count starting at the value configured
          in the address file.
  - count: specifies the number of bytes to be read or read
  - help: prints help text

Change-Id: I5c90fe8e77271dd2fa369c6d6fe33f01ee712039
Signed-off-by: David Keitel <dkeitel@codeaurora.org>