ARM: tegra: add DT entry for KBC controller

NVIDIA's Tegra SoCs have the matrix keyboard controller which
supports 16x8 type of matrix. The number of rows and columns
are configurable.

Add DT entry for KBC controller.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
[swarren: added clocks property]
Signed-off-by: Stephen Warren <swarren@nvidia.com>