uart: ns16550: Enable clock if defined

Currently, ns16550 allows the user to specify a clock attribute in the
Device Tree. This node is use to retrieve the internal frequency of the
hardware block.

However, ns16550 expects the clock is already initialised and it doesn't
try to enable it. Let's fix that issue.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>