dt-binding: add Tegra WDT dt-binding document

dt-binding: add Tegra WDT dt-binding document

This is based on below commits:
    commit 9c20740436f06f4b5af4b7805df25ef0dec5dfb2
    Author: Allen Yu <alleny@nvidia.com>
    Date:   Wed Apr 29 15:10:51 2015 +0800

    ARM: tegra: add DT properties for tegra WDT <Allen Yu>

    Add "interrupts" and "nvidia,expiry-count" DT properties
    for tegra WDT to better support enable WDT on probe.

    Bug 200100035

    commit 7224891dff785257defbadfc5fc88257ddb49426
    Author: Robert Shih <rshih@nvidia.com>
    Date:   Mon Feb 9 17:34:59 2015 +0800

    dt-binding: add Tegra WDT dt-binding document <Robert Shih>

Bug 200314562

Change-Id: I3a7409571f80c0f815164d1b371d76e86d61025b
Signed-off-by: Allen Yu <alleny@nvidia.com>
Signed-off-by: Robert Shih <rshih@nvidia.com>
Signed-off-by: Shreshtha SAHU <ssahu@nvidia.com>
Reviewed-on: https://git-master/r/1515090
GVS: Gerrit_Virtual_Submit
Reviewed-by: Bibek Basu <bbasu@nvidia.com>
Tested-by: Bibek Basu <bbasu@nvidia.com>