dt-bindings: qpnp-fg-gen4: add support to read/write from nvmem

Add support to read and write FG parameters to SDAM through
nvmem framework. "fg_sdam" should be the nvmem name if specified.

Change-Id: I4d0ddcbcad0834704c8b497c1f43503b8518f971
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>