dt-bindings: timer: Add renesas,em-sti bindings

Document Device Tree bindings for the Renesas EMMA Mobile System Timer.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200519081101.28973-1-geert+renesas@glider.be