amd/sb700: Move HAVE_HARD_RESET to southbridge

amd/sb700: Move HAVE_HARD_RESET to southbridge

No in-tree board using that chipset has it not selected, so move
selection from boards to southbridge.

Change-Id: I7a7a1919b7a555156b8da21e8db7dd8f682d68e1
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/661
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)