phy: msm: usb: Find USB power supply as per requirement

Currently we are trying to find USB power supply registered by
PMIC during msm_otg_probe. If we fail to get it, then we defer
the probe. On some targets, this can lead to a deadlock situation
where PMI is looking for dpdm-supply node before it registers
USB power supply.
Fix this by looking for power supply as and when required during
runtime.

Change-Id: I6f5f84719ad4ebca1ffd728cdbb44eb2b7b82a37
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>