include: usbc: doc: Group all USB-C API in same doxygen Group

Put every header defined in include/zephyr/drivers/usb_c/ into the same
doxygroup (`usb_type_c`) so as to not clutter the top-level
`io_interfaces` group with multiple USB-C related groups.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>