cpu/intel/sandybridge: Use enum for ACPI C states

Also remove the now unnecessary comments from the devicetree.

Change-Id: Iebbe12fd413b7a2eb1078a579e194eba821ada7c
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69292
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/northbridge/intel/sandybridge/chipset.cb

@@ -8,9 +8,9 @@ chip northbridge/intel/sandybridge
 			device lapic 0 on end
 			device lapic 0xacac off end
 
-			register "acpi_c1" = "1"	# ACPI(C1) = MWAIT(C1)
-			register "acpi_c2" = "3"	# ACPI(C2) = MWAIT(C3)
-			register "acpi_c3" = "5"	# ACPI(C3) = MWAIT(C7)
+			register "acpi_c1" = "CPU_ACPI_C1"
+			register "acpi_c2" = "CPU_ACPI_C3"
+			register "acpi_c3" = "CPU_ACPI_C7"
 		end
 	end