usb: dwc3: Add support for variable number of GSI endpoints

Currently, the gadget driver fixes the number of GSI IN EPs to 2
and OUT EPs to 1. But some targets have more HW accelerated EPs
available for use.
Add new property 'num-gsi-eps' from glue driver to dwc3 child
node to initialize variable number of GSI EPs based on devicetree
property 'qcom,num-gsi-evt-buffs'.

Change-Id: Iab99a8398440fc1b44563e4080c39868835f932b
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>