BCM270X_DT: Add pi3-miniuart-bt DT overlay

Switch Pi3 Bluetooth function to use the mini-UART (ttyS0) and restore
UART0/ttyAMA0 over GPIOs 14 & 15. Note that this may reduce the maximum
usable baudrate.

It is also necessary to edit /lib/systemd/system/hciuart.server and
replace ttyAMA0 with ttyS0.

If cmdline.txt uses the alias serial0 to refer to the user-accessable port
then the firmware will replace with the appropriate port whether or not
this overlay is used.

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