drivers: mfd: Set ANC gain register as volatile

ANC register values are not correct, so add ANC0,
ANC1 registers to volatile register list.

CRs-Fixed: 1017287
Change-Id: I40e12b9ce8d6751c0a6dc0bd3c780b92c499b7dc
Signed-off-by: Surendar karka <sukark@codeaurora.org>