platform/x86: system76_acpi: Guard System76 EC specific functionality

Certain functionality or its implementation in System76 EC firmware may
be different to the proprietary ODM EC firmware. Introduce a new bool,
`has_open_ec`, to guard our specific logic. Detect the use of this by
looking for a custom ACPI method name used in System76 firmware.

Signed-off-by: Tim Crawford <tcrawford@system76.com>
Link: https://lore.kernel.org/r/20211222185154.4560-1-tcrawford@system76.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>