clk: imx: imx8mq: mark sys1/2_pll as fixed clock

According Architecture definition guide, SYS1_PLL is fixed at
800MHz, SYS2_PLL is fixed at 1000MHz, so let's use imx_clk_fixed
to register the clocks and drop code that could change the rate.

Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>