dt-bindings: move various timer bindings to timer/ directory

Bindings are supposed to be organized by device class/function. Move
bindings for various timers to timer/ binding directory.

Cc: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>