dt-bindings: Renamed hte directory to timestamp

Renamed hte dt binding directory to timestamp according review comment.
Addressed minor comment about having HTE acronym first in the common dt
binding document.

The change reflects above renaming in MAINTAINERS files too.

Signed-off-by: Dipen Patel <dipenp@nvidia.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>