acpi/acpigen.c: Add a comment to deprecate acpigen_write_processor()

ACPI Revision 6.0 deprecates Processor keyword, so use
acpigen_write_processor_device() instead.

Change-Id: I31626f4e323dd9053a63c0f5e89d1685103e4bd4
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/72487
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/acpi/acpigen.c

@@ -395,6 +395,7 @@ void acpigen_write_processor_namestring(unsigned int cpu_index)
 	acpigen_emit_namestring(buffer);
 }
 
+/* Processor() operator is deprecated as of ACPI 6.0, use Device() instead. */
 void acpigen_write_processor(u8 cpuindex, u32 pblock_addr, u8 pblock_len)
 {
 /*