usb: dwc3: keystone: Add PM_RUNTIME Support to DWC3 Keystone USB driver

For 66AK2Gx there is a requirement to use PM Runtime to properly manage
clocks and the power domains. Therefore, add PM runtime support. Remove
legacy clock api's calls since other users of this driver worked without
these clock apis calls.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>