BCM270X_DT: Add midi-uart1 overlay

Add a scaler to the ttyS0 clock so that requesting 38400 baud results
in an approximately 31250 baud signal. This is analagous to
midi-uart0, except for ttyS0, which may be useful on Pi3 and also
may avoid an issue with ttyAMA0 failing to synchronise to an active
data stream.

See: https://www.raspberrypi.org/forums/viewtopic.php?f=107&t=183860



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