ACPI: Rename acpi_scan_device_not_present() to be about enumeration

acpi_scan_device_not_present() is called when a device in the
hierarchy is not available for enumeration. Historically enumeration
was only based on whether the device was present.

To add support for only enumerating devices that are both present
and enabled, this helper should be renamed. It was only ever about
enumeration.

Signed-off-by: James Morse <james.morse@arm.com>