usb: gadget: udc: Add Synopsys UDC Platform driver

This patch adds platform driver support for Synopsys UDC.

A new driver file (snps_udc_plat.c) is created for this purpose
where the platform driver registration is done based on OF
node.

Currently, UDC integrated into Broadcom's iProc SoCs (Northstar2
and Cygnus) work with this driver.

New members are added to the UDC data structure for having platform
device support along with extcon and phy support.

Kconfig and Makefiles are modified to select platform driver for
compilation.

Signed-off-by: Raviteja Garimella <raviteja.garimella@broadcom.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>