ASoC: apq8064: Add run-time PM support for voice over Bluetooth.

When external modem is paired with the apps processor, voice call
over BT uses Slimbus to transfer voice packets from the modem to
the application processor. Enable run-time PM so that Slimbus
driver keeps the clocks enabled in the BT usecase.

CRs-fixed: 368527
Change-Id: Ic4653e304bdef7ea6303c89918ce4cfa195ba968
Signed-off-by: Neema Shetty <nshetty@codeaurora.org>