ACPI: Export acpi_get_first_physical_node() to modules

This function will be needed by the Thunderbolt driver when it parses
ACPI description for linking tunneled ports to the Thunderbolt
controller device.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>