clk: imx8mq: add PLL monitor output

The PLL monitor is mentioned as a debug feature in the reference manual,
but there are some boards that use this clock output as a reference clock
for board level components. Add support for those clocks in the clock
driver, so this clock output can be used properly.

Note that the VIDEO1, GPU and VPU mux inputs are rotated compared to the
description in the reference manual. The order in this patch has been
empirically validated.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>