boards: opta: external flash and BLE support

This set of changes adds support for QSPI-based external flash and
Bluetooth to the device tree. This make it possible to correctly build and
execute the fatfs and several Bluetooth samples out of the box.

Also added a function to read the external flash OTP to extract
information about the Opta model and hardware features and a second
function to retrieve the "official" Opta serial number.

Signed-off-by: Federico Di Gregorio <fog@dndg.it>