ARM: dts: Add Bluetooth nodes for Raspberry Pi

Add device tree nodes for Bluetooth on supported Raspberry Pi boards.
It's disabled by default and can be enabled by `krnbt=on` dtparam. It's
an alternative way of configuring Bluetooth, as compared to hciattach or
btattach. When the dtparam is enabled, the Bluetooth driver is probed
automatically and doesn't require any additional bring-up scripts.

Note that Raspberry Pi 3 B rev 1.2 doesn't have the required hardware
flow control pins of UART0 connected to the Bluetooth module, so the
user should decrease the baudrate by passing `krnbt_baudrate=921600`
dtparam to make it more stable. It resembles the behavior of the btuart
script from Raspbian.

The miniuart-bt overlay was modified to support Bluetooth probing with
device tree, too. It's disabled by default and can be enabled by
`krnbt=on` parameter of the miniuart-bt overlay.

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>