thermal: int340x: Add keep alive response method

When firmware requests keep alive response, send an event to user space
to confirm by using imok sysfs entry.

Create a new sysf entry called "imok". User space can write an integer,
which results in execution of IMOK ACPI method of INT3400 thermal zone
device. This results in sending response to firmware request for keep
alive.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200915223650.406046-4-srinivas.pandruvada@linux.intel.com