rpi: BCM2837 and Raspberry Pi 3 32-bit support

The Raspberry Pi 3 contains a BCM2837 SoC. The BCM2837 is a BCM2836 with
the CPU complex swapped out for a quad-core ARMv8. This can operate in 32-
or 64-bit mode. 32-bit mode is the current default selected by the
VideoCore firmware on the Raspberry Pi 3. This patch adds a 32-bit port of
U-Boot for the Raspberry Pi 3.

>From U-Boot's perspective, the only delta between the RPi 2 and RPi 3 is a
change in usage of the SoC UARTs. On all previous Pis, the PL011 was the
only UART in use. The Raspberry Pi 3 adds a Bluetooth module which uses a
UART to connect to the SoC. By default, the PL011 is used for this purpose
since it has larger FIFOs than the other "mini" UART. However, this can
be configured via the VideoCore firmware's config.txt file. This patch
hard-codes use of the mini UART in the RPi 3 port. If your system uses the
PL011 UART for the console even on the RPi 3, please use the RPi 2 U-Boot
port instead. A future change might determine which UART to use at
run-time, thus allowing the RPi 2 and RPi 3 (32-bit) ports to be squashed
together.

The mini UART has some limitations. One externally visible issue in the
BCM2837 integration is that the UART divides the SoC's "core clock" to
generate the baud rate. The core clock is typically variable, and under
control of the VideoCore firmware for thermal management reasons. If the
VC FW does modify the core clock rate, UART communication will be
corrupted since the baud rate will vary from the expected value. This was
not an issue for the PL011 UART, since it is fed by a fixed 3MHz clock. To
work around this, the VideoCore firmware can be told not to modify the SoC
core clock. However, the only way this can happen and be thermally safe is
to limit the core clock to a low/minimum frequency. This leaves
performance on the table for use-cases that don't care about a UART
console. Consequently, use of the mini UART console must be explicitly
requested by entering the following line into config.txt:

    enable_uart=1

A recent version of the VC firmware is required to ensure that the mini
UART is fully and correctly initialized by the VC FW; at least
firmware.git 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>