mb/asus/p8x7x-series: Hide nonexistent IVB devices

Hide peg12, dev4, and peg60 devices to squelch leftover devices
warning seen with p8z77-m, p8z77-v and p8z77-v_le_plus:

[WARN ]  PCI: Leftover static devices:
[WARN ]  PCI: 00:00:01.1
[WARN ]  PCI: 00:00:01.2
[WARN ]  PCI: 00:00:04.0
[WARN ]  PCI: 00:00:06.0
[WARN ]  PCI: Check your devicetree.cb.

No board in this family can do the 8/4/4 PCIe lane split to
require the peg12 bridge, or implemented dev4 at all.

Only p8c_ws wired up the 4 extra Xeon PCIe lanes peg60 is supposed
to cover, and its overridetree already enables it.

Therefore, be proactive and hide these from the rest of the family.

Change-Id: I24234e6b77a9effc577c8e22c77bb9896b983b7f
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/86185
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

src/mainboard/asus/p8x7x-series/devicetree.cb

@@ -9,7 +9,10 @@ chip northbridge/intel/sandybridge
 
 	device domain 0 on
 		device ref peg10 on  end # PCIEX16_1
+		device ref peg12 hidden end
 		device ref igd on  end
+		device ref dev4 hidden end
+		device ref peg60 hidden end
 
 		chip southbridge/intel/bd82x6x
 			register "sata_interface_speed_support" = "0x3"