usb: dwc2: gadget: Add new core parameter for low speed

Added new core param for low speed, which can be used only when SNPSID
is equal to DWC2_CORE_FS_IOT. When LS mode is enabled, we are
restricting ep types and providing to upper layer only INTR and CTRL
endpoints.

Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>