ARM: Orion: UART: Get the clock rate via clk_get_rate().

Let the machine pass to the platform which clock is used by the uart.
Enable the clock and use clk_get_rate() to determine its rate.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Jamie Lentin <jm@lentin.co.uk>
Signed-off-by: Mike Turquette <mturquette@linaro.org>