Bluetooth: Enable support for retention power mode

Add the support for retention mode. BT host enters this mode from
once the BT is ON. In this mode, LDOs are still enabled but MSM
votes for voltage and current are set to 0. SW_CTRL from BT SoC
takes control of voting for these  LDOs.

Change-Id: If595c2cdc6e38a46ed78045a0373286ff97cfe5b
Signed-off-by: Umesh Vats <uvats@codeaurora.org>