usb: gadget: fsl: properly remove remnant of MXC support

usb: gadget: fsl: properly remove remnant of MXC support

Commit dadd7e952f53 ("usb: gadget: fsl_mxc_udc: Remove the driver")
didn't remove all the MXC related stuff which can cause build problem
for LS1021 when enabled again in Kconfig.  This patch remove all the
remnants.

Reviewed-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Li Yang <leoyang.li@nxp.com>
Link: https://lore.kernel.org/r/20210612003128.372238-1-leoyang.li@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>