ARM: tegra: add device tree for SHIELD

NVIDIA SHIELD is a portable Android console containing a Tegra 4 SoC with
2GB RAM and a 720p panel.

The following hardware is enabled by this device tree: UART, eMMC, USB
(needs external power), PMIC, backlight, joystick, SD card, GPIO keys.

DSI panel, HDMI output, charger, self-powered USB, audio, wifi bluetooth
are not supported yet but might be by future patches (likely in that
order).

Touch panel and sensors will probably never be supported.

Initrd addresses are hardcoded to match the static values used by the
bootloader, since it won't add them for us. All the same, a kernel
command-line is provided to replace the one passed by the
bootloader which is filled with garbage.

NVIDIA SHIELD is typically booted with an appended DTB to avoid
modifications made by the bootloader.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
[swarren, fixed gpio-keys child node sort order, patch description]
Signed-off-by: Stephen Warren <swarren@nvidia.com>