platform: Add ARM64 platform directory

Some ARM64 based laptops and computers require vendor/board specific
drivers for their embedded controllers. Even though usually the most
important functionality of those devices is implemented inside ACPI,
unfortunately Linux doesn't currently have great support for ACPI on
platforms like Qualcomm Snapdragon that are used in most ARM64 laptops
today. Instead Linux relies on Device Tree for Qualcomm based devices
and it's significantly easier to reimplement the EC functionality in
a dedicated driver than to make use of ACPI code.

This commit introduces a new platform/arm64 subdirectory to give a
place to such drivers for EC-like devices.

A new MAINTAINERS entry is added for this directory. Patches to files in
this directory will be taken up by the platform-drivers-x86 team (i.e.
Hans de Goede and Ilpo Järvinen) with additional review from Bryan
O'Donoghue to represent ARM64 maintainers.

Signed-off-by: Nikita Travkin <nikita@trvn.ru>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20240315-aspire1-ec-v5-2-f93381deff39@trvn.ru
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>