clocksource: vf_pit_timer: Support shutdown mode

In order to avoid waking up the system in a low power mode, the
clocksource should not generate interrupts anymore. Disable the PIT
timer interrupt when changing into the CLOCK_EVT_MODE_SHUTDOWN mode.

[dlezcano] : remove superfluous empty line

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Bill Pringlemeir <bpringlemeir@nbsps.com>