BCM270X_DT: Update UART0 clock-frequency

The Raspberry Pi firmware now sets the UART0 clock frequency to
48MHz (was 3MHz), to allow for baudrates up to 3MHz. This value
is overwritten by the firmware, but alternative loaders such
as U-Boot ignore the firmware-provided DTB. And besides, the
default ought to be correct.

See: https://github.com/raspberrypi/linux/issues/1732



Signed-off-by: Phil Elwell <phil@raspberrypi.org>