usb: xhci: Move extcaps related macros to respective header file

DWC3 driver needs access to XHCI Extended Capabilities registers to
read number of usb2 ports and usb3 ports present on multiport controller.
Since the extcaps header is sufficient to parse this info, move port_count
related macros and structure from xhci.h to xhci-ext-caps.h.

Signed-off-by: Krishna Kurapati <quic_kriskura@quicinc.com>
Link: https://lore.kernel.org/r/20230828133033.11988-4-quic_kriskura@quicinc.com
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>