armv8/fsl-lsch3: Add fdt-fixup for clock frequency of the DUART nodes

This patch adds the fdt-fixup logic for the clock frequency of the
NS16550A related device tree nodes.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>