rpi: add Raspberry Pi 3 board ID

This allows U-Boot to known the name of the board.

The existing rpi_2_defconfig can operate correctly on the Raspberry Pi 3
in 32-bit mode /if/ you have configured the firmware to use the PL011 UART
as the console UART (the default is the mini UART). This requires two
things:
a) config.txt should contain dtoverlay=pi3-miniuart-bt
b) You should run the following to tell the VC FW to process DT when
booting, and copy u-boot.bin.img (rather than u-boot.bin) to the SD card
as the kernel image:

   path/to/kernel/scripts/mkknlimg --dtok u-boot.bin u-boot.bin.img

This works as of firmware.git commit 046effa13ebc "firmware: arm_loader:
emmc clock depends on core clock See:
https://github.com/raspberrypi/firmware/issues/572

".

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Tom Rini <trini@konsulko.com>