rpi3: Detect board revision ...

Implement VideoCore mailbox interface driver and use it to get the board
revision identifier.

For now it is only used to print the model for debug purposes.

This wiki contains the documentation of the mailbox interface:

https://github.com/raspberrypi/firmware/wiki

Change-Id: I11943b99b52cc1409f4a195ebe58eb44ae5b1d6c
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>