MAINTAINERS: Add ARM64-specific ACPI maintainers entry

The ARM64 architecture defines ARM64 specific ACPI bindings to
configure and set-up arch specific components. To simplify
code reviews/updates and streamline the maintainership structure
supporting the arch specific code, a new arm64 directory was created in
/drivers/acpi, to contain ACPI code that is specific to ARM64
architecture.

Add the ARM64-specific ACPI maintainers entry in MAINTAINERS for
the newly created subdirectory and respective code content.

Lorenzo Pieralisi will be in charge of submitting and managing
the pull requests on behalf of all maintainers listed.

Link: http://lkml.kernel.org/r/1603704.EGiVTcCxLR@vostro.rjw.lan


Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Hanjun Guo <hanjun.guo@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>