serial: actions: add compatible string

This patch adds "actions,owl-uart" string to the owl uart driver. It
is also defined in Linux kernel.

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>