acpi: add STA function to return external variable

This change adds support function to add STA function which returns an
external variable.

Change-Id: I31755a76ee985ee6059289ae194537d531270761
Signed-off-by: Sugnan Prabhu S <sugnan.prabhu.s@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42245
Reviewed-by: Rizwan Qureshi <rizwan.qureshi@intel.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/acpi/acpigen.c

@@ -742,6 +742,17 @@ void acpigen_write_STA(uint8_t status)
 	acpigen_pop_len();
 }
 
+void acpigen_write_STA_ext(const char *namestring)
+{
+	/*
+	 * Method (_STA, 0, NotSerialized) { Return (ext_val) }
+	 */
+	acpigen_write_method("_STA", 0);
+	acpigen_emit_byte(RETURN_OP);
+	acpigen_emit_namestring(namestring);
+	acpigen_pop_len();
+}
+
 /*
  * Generates a func with max supported P-states.
  */