can: rockchip_canfd: add support for CAN_CTRLMODE_LOOPBACK

Add support for loopback mode.

Tested-by: Alibek Omarov <a1ba.omarov@gmail.com>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/20240904-rockchip-canfd-v5-19-8ae22bcb27cc@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>