mfd: wcd9335-regmap: Mark Vbat registers as volatile

Vbat adc, gain related registers are not updated by the
driver and are read only. Mark these registers as volatile,
to get proper values when the registers are read.

Change-Id: Icef27deb9459a3193110007a78ad1a84283f4965
Signed-off-by: Venkata Narendra Kumar Gutta <vgutta@codeaurora.org>
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>