armv8/fsl-lsch3: Use correct compatible for serial clock fixup

The serial nodes in the fsl-lsch3 device trees have compatible =
"fsl,ns16550", "ns16550a" -- so don't look for "ns16550".

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: York Sun <yorksun@freescale.com>