MIPS: pistachio: Allow to enable the external timer based clocksource

This commit introduces a new config, so the user can choose to enable
the General Purpose Timer based clocksource. This option is required
to have CPUFreq support.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
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: Andrew Bresticker <abrestic@chromium.org>
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>
Cc: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Patchwork: http://patchwork.linux-mips.org/patch/10887/


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