Break out the domain allocation code into a separate function.

This is preparation for future code sharing.
Signed-off-by: NMagnus Damm <damm+renesas@opensource.se>
Reviewed-by: NJoerg Roedel <jroedel@suse.de>
Reviewed-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NJoerg Roedel <jroedel@suse.de>