serial: ns16550: Handle -ENOENT when requesting clock

When calling clk_get_by_index(), fall back to the legacy method of
getting the clock if -ENOENT is returned.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>