mfd: wcd934x: make regmap cache only during SSR

During SSR or when device goes down doing any transactions on the bus might
cause lockups, so set regmap to be in cache only mode during this time.

Fixes: 6ac7e4d7 ("mfd: wcd934x: Add support to wcd9340/wcd9341 codec")
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>