clk: imx8mm: Fix typo of pwm3 clock's mux option #4

i.MX8MM has no sys3_pll2_out clock, PWM3 clock's mux option #4
should be sys_pll3_out, sys3_pll2_out is a typo, fix it.

Fixes: ba5625c3 ("clk: imx: Add clock driver support for imx8mm")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>