ARM: tegra: define valid function names in DT document

List valid values for the property "nvidia,function" in
DT binding document of Tegra pincontrol.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
[swarren, whitespace fixes, grammar consistency fixes, section
ordering fixes]
Signed-off-by: Stephen Warren <swarren@nvidia.com>