configs: k2g_evm_defconfig: disable XHCI_DWC3 and enable KEYSTONE_USB_PHY

KEYSTONE_USB_PHY is now required for proper USB operations.
XHCI_DWC3 can be disabled as the xhci-dwc3 is not used anymore
USB_DWC3_GADGET can also be removed, it is now selected automatically.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>