ACPI / PMIC: Add code reviewers to MAINTAINERS

Andy and Mika review code changes under drivers/acpi/pmic/ on
a regular basis and I rely on their help with that, so add them
as code reviwewers for that part of the kernel.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>