Add support for A-Open DXPL Plus-U motherboard

Add support for A-Open DXPL Plus-U motherboard

This is an old (pre-2005) entry-level server mainboard. The code
is adapted from mainboard/intel/xe7501devkit.

Featured chips:
 - Dual socket604
 - E7505 northbridge
 - 82801DB southbridge (with EHCI debug port)
 - 82870p2 PCI-X bridge
 - LPC47M102S-MC super-io
 - 512kB FWH flash (flashrom does the job well)

What works:
 - Dual-Xeon P4/HT boot with microcode update
 - RAM: registered ECC DDR266 in dual-channel
 - PCI-X slot interrupts with ACPI and I/O apic
 - On-board PCI-X GbE and SCSI
 - ACPI power-off and wakeup with PME#

Notes :
 - Current ACPI is more or less a mess
 - Interrupts do not route correctly with PIRQ
 - MP-table is not implemented
 - Issues with reboots remain (cold and warm)
 - Many superio devices are disabled by default
 - Audio codec is not investigated

Change-Id: I02d18c83f485a09ada65dde03bcc86e9163f2011
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/303
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>