misc: liquid_dock: Make dock_enable GPIO optional

The MSM8994 LiQUID docking station does not have a
pin for a dock_enable GPIO, as the USB and Ethernet
ports are always powered via the DC source. Make
this GPIO optional.

As the power to the hub and Ethernet ports may now
be always-on, also make sure to assert the reset lines
prior to adding the USB host controller. This way the
deassertion of the resets will happen after the USB
stack is fully initialized to kick off enumeration.

Change-Id: I864ebcfa94d58e741e7378db0c24fdc6220dc18f
Signed-off-by: Jack Pham <jackp@codeaurora.org>