ARM: tegra: add p2771-0000 board support

P2771-0000 is a P3310 CPU board married to a P2597 I/O board. The
combination contains SoC, DRAM, eMMC, SD card slot, HDMI, USB micro-B
port, Ethernet, USB3 host port, SATA, PCIe, and two GPIO expansion
headers.

Currently, due to U-Boot's level of support for Tegra186, the only
features supported by U-Boot are the console UART and the on-board eMMC.
Additional features will be added over time.

U-Boot has so far been tested by replacing the kernel image on the device
with a U-Boot binary. It is anticipated that U-Boot will eventually
replace the CCPLEX bootloader binary, as on previous chips. This hasn't
yet been tested.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>