Documentation: dt: Add Pistachio SoC general purpose timer binding document

Add a device-tree binding document for the clocksource driver provided
by Pistachio SoC general purpose timers.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Reviewed-by: Andrew Bresticker <abrestic@chromium.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: devicetree@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: James Hartley <James.Hartley@imgtec.com>
Cc: Govindraj Raja <Govindraj.Raja@imgtec.com>
Cc: Damien Horsley <Damien.Horsley@imgtec.com>
Cc: James Hogan <James.Hogan@imgtec.com>
Cc: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Patchwork: https://patchwork.linux-mips.org/patch/10783/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>