arch/x86/acpigen: Constify fieldlist parameter to acpigen_write_field

acpigen_write_field() does not need to modify the fieldlist
parameter. Thus, this change makes this parameter as const.

Change-Id: I94688913cee8948f42ae5e184f2d24264876648d
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40784
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/acpigen.c

@@ -514,7 +514,7 @@ static void acpigen_write_field_name(const char *name, uint32_t size)
  *		PMCS,   2
  *	}
  */
-void acpigen_write_field(const char *name, struct fieldlist *l, size_t count,
+void acpigen_write_field(const char *name, const struct fieldlist *l, size_t count,
 			 uint8_t flags)
 {
 	uint16_t i;

src/arch/x86/include/arch/acpigen.h

@@ -426,7 +426,7 @@ void acpigen_write_release(const char *name);
  * Generate ACPI AML code for Field
  * This function takes input region name, fieldlist, count & flags.
  */
-void acpigen_write_field(const char *name, struct fieldlist *l, size_t count,
+void acpigen_write_field(const char *name, const struct fieldlist *l, size_t count,
 			 uint8_t flags);
 /*
  * Generate ACPI AML code for IndexField