mb/*,soc/intel: drop the obsolete dt option speed_shift_enable

The dt option `speed_shift_enable` is obsolete now. Drop it.

Change-Id: I5ac3b8efe37aedd442962234478fcdce675bf105
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46462
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Reviewed-by: Frans Hendriks <fhendriks@eltan.com>

src/soc/intel/skylake/chip.h

@@ -461,8 +461,7 @@ struct soc_intel_skylake_config {
 	 */
 	u8 HeciEnabled;
 	u8 PmTimerDisabled;
-	/* Intel Speed Shift Technology */
-	u8 speed_shift_enable;
+
 	/*
 	 * Enable VR specific mailbox command
 	 * 000b - Don't Send any VR command