ARM: dt: tegra: add Whistler device tree file

Whistler is a highly configurable Tegra evaluation and development board.
This change adds support for the following specific configuration:

E1120 motherboard
E1108 CPU board
E1116 PMU board

The motherboard configuration switches are set as follows:
SW1=0 SW2=0 SW3=5
S1/S2/S3/S4 all on, except S3 7/8 are off.

Other combinations of daugher boards may work to varying degrees, but will
likely require some SW adjustment.

Signed-off-by: Stephen Warren <swarren@nvidia.com>