sb/intel/i82801gx: Detect if the southbridge supports AHCI

This automatically detects whether the southbridge supports AHCI.
If AHCI support is selected it will be used unless "sata_no_ahci" is
set in the devicetree to override the behavior.

Change-Id: I8d9f4e63ae8b2862c422938f3103c44e761bcda4
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/30822
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/mainboard/getac/p470/devicetree.cb

@@ -57,7 +57,6 @@ chip northbridge/intel/i945
                         register "ide_legacy_combined" = "0x1"
                         register "ide_enable_primary" = "0x1"
                         register "ide_enable_secondary" = "0x0"
-                        register "sata_ahci" = "0x0"
 
 			register "c3_latency" = "85"
 			register "docking_supported" = "1"