dt-bindings: serial: Add DT binding entry for UART sampling rate

Add DT binding entry for serial UART driver to specify what sampling
rate sequencer is running. This flag determines clock rate to meet
the respective sampling rate.

Change-Id: I9d3e9cb0e9d0ee8dbeb0f639d198dabb8933a840
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>