clk: imx7ulp: Make sure earlycon's clock is enabled

Earlycon's clock could be disabled during kernel boot up,
if earlycon is enabled and its clock is gated, then kernel
boot up will fail. Make sure earlycon's clock is enabled
during kernel boot up.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>