pinctrl: tegra: Add devicetree binding document for Tegra124

This device tree binding document describes the Tegra124 pincontrol
DT bindings. This document lists all valid properties, names, mux
options of Tegra124 pins.

Changes from V1:
- Referred the dt-binding header file on describing the nodes.

Changes from V2:
- Rewording reg properties.
- drop drv_type as it is not applicable.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>