clk: imx8mm: add clkout1/2 support

clkout1 and clkout2 allow to supply clocks from the SoC to the board,
which is used by some board designs to provide reference clocks.

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>