usb: dwc2: Add Interpacket Gap(IPG) feature support

Added GHWCFG4_IPG_ISOC_SUPPORTED and DCFG_IPG_ISOC_SUPPORDED
bits definitions to enable/disable IPG feature.

Added ipg_isoc_en core parameter which will indicate IPG support
enable/disable and initialize it.

Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>