clk: imx8m: Add GIC clock

This is documented in the reference manuals as GIC_CLK_ROOT.

In some out-of-tree DVFS scenarios the gic clock can end up as the only
user of sys_pll2 so if we don't define the gic clk explicitly it might
be turned off.

This applies to both 8mq and 8mm: same clk register but diffferent
parents.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>