soc/amd: Clarify ACPI _PRT entry generation

The reference to a constant FCH IOAPIC interrupt count used
with GNB IOAPIC was a bit obscure.

Change-Id: I2d862e37424f9fea7f269cd09e9e90056531b643
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74314
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/arch/x86/include/arch/ioapic.h

@@ -5,7 +5,6 @@
 
 #define IO_APIC_ADDR	0xfec00000
 #define VIO_APIC_VADDR	((u8 *)IO_APIC_ADDR)
-#define IO_APIC_INTERRUPTS 24
 
 #ifndef __ACPI__