USB: ehci-msm2: Add support to RESET external HUB using RESET_N GPIO

Some external USB hubs connected to USB host may require to be
reset using RESET_N signal connected to a GPIO. Add support for
the same by passing GPIO number using device-tree.

Change-Id: I41e0138f90f0718e2a88bb141b3f2a76b87389e8
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>