arm64: juno: Fix misleading name of UART reference clock

The UART reference clock speed is 7273.8 kHz, not 72738 kHz.

Dots aren't usually used in node names even though ePAPR permits
them.  However, this can easily be avoided by expressing the
frequency in Hz, not kHz.

This patch changes the name to refclk7273800hz, reflecting the
actual clock speed.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Olof Johansson <olof@lixom.net>
(cherry picked from commit 78d84bc3734c2566dbba09baae2414734661ed6a)
Signed-off-by: Kevin Hilman <khilman@linaro.org>