Bluetooth: hci_sync: Add fallback-bd-address prop

The kernel Bluetooth framework understands that devices may not
be programmed with valid Bluetooth addresses. It also has the ability
to override a Bluetooth address with the value of the local-bd-address
DT property, but it ignores the validity of the existing address when
doing so.

Add a new boolean property, fallback-bd-address, which indicates that
the given local-bd-address property should only be used if the device
does not already have a valid BDADDR.

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