[U-Boot] [U-Boot,2/2] rpi: Add identifier for the new RPi3 B+

[Alexander Graf]

> The Raspberr Pi Foundation released a new RPi3 version which we want
> to detect as well, so we can enable ethernet on it and know the correct
> device tree file name.
> 
> Add an identifier for it.
> 
> Signed-off-by: Alexander Graf <agraf at suse.de>

Thanks, applied to efi-next

Alex