mfd: wcd9xxx: Get list of supplies from device tree

The wcd9xxx codec driver doesn't know what supplies need to be enabled for
different hardwares.  Therefore currently driver maintains list of supplies
for each individual codec hardwares which isn't good practice.
Get name of supplies from device tree as device tree already knows what
supplies are configured to get rid of hardcoded supply lists from driver.

Change-Id: Id5640c0f4636c7b7a6c79f14457adb52062095f4
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>