CLOCKSOURCE: mips-gic: Enable the clock before using it

For the clock to be used (e.g. get its rate through clk_get_rate)
it should be prepared and enabled first.

Also, while the clock is enabled the driver must hold a reference to it,
so let's remove the call to clk_put.

Reviewed-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.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/10779/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>