ARM: tegra: note that p2371-2180 is Jetson TX1

p2371-2180 is the engineering board name for the Jetson TX1 developer
kit. Update Kconfig description and help text to make this obvious to
everyone.

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