clk: imx: Add fractional PLL output clock

This is a new fractional clock type introduced on i.MX8.

The description of this fractional clock can be found here:

https://www.nxp.com/docs/en/reference-manual/IMX8MDQLQRM.pdf#page=834



Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>