dt-binding: tegra: add timer source documentation

dt-binding: tegra: add timer source documentation

Add documentation of possible methods to provide WDT timer sources,
their priorities of selection and restriction in usage of each
method.

Bug 200314562

Change-Id: Ib19c9962e181301b9b3b373c7e079c211a6e09e8
Signed-off-by: Shreshtha SAHU <ssahu@nvidia.com>
Reviewed-on: https://git-master/r/1515193
(cherry picked from commit 9e3217293821dd9b147918e747d33073e0785ed4)
Reviewed-on: https://git-master.nvidia.com/r/1506166
GVS: Gerrit_Virtual_Submit
Reviewed-by: Krishna Yarlagadda <kyarlagadda@nvidia.com>
Reviewed-by: Bibek Basu <bbasu@nvidia.com>