docs: add Tegra186 information to nvidia-tegra.rst
docs: add Tegra186 information to nvidia-tegra.rst

This patch adds information about the Tegra186 platforms to the
docs.

Change-Id: I69525c60a0126030dc15505ec7f02ccf8250be6f
Signed-off-by: Varun Wadekar <517ae08f01cae398dbba5bef5ba1750f384c6daf@nvidia.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware