MAINTAINERS / ACPICA: Add ACPICA information to MAINTAINERS

ACPICA (ACPI Component Architecture) is an external project that
some ACPI kernel code, including the AML interpreter, is derived
from.  That kernel code is generated automatically out of the
original upstream ACPICA sources and therefore, as a general rule,
all changes to it have to go through the ACPICA upstream.

Add ACPICA information to MAINTAINERS to provide the upstream
ACPICA maintainers contact information and pointers to the original
ACPICA Web site and source code.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>