PM / devfreq: tegra: Properly disable interrupts

There is no guarantee that interrupt handling isn't running in parallel
with tegra_actmon_disable_interrupts(), hence it is necessary to protect
DEV_CTRL register accesses and clear IRQ status with ACTMON's IRQ being
disabled in the Interrupt Controller in order to ensure that device
interrupt is indeed being disabled.

Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>