mfd: wcd9xxx: Get micbias cap mode config from device tree

Parse device tree to get micbias external capacitor mode to fill it into
platform data.  External capacitor mode needs to be passed to codec drvier
so that codec driver can configure it.

Change-Id: I5a2137d7ede96a324ad43e48b8642bbf8ed2118e
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>