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The dt option `speed_shift_enable` is obsolete now.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/elkhartlake/chip.h

@@ -149,8 +149,6 @@ struct soc_intel_elkhartlake_config {
 	/* HeciEnabled decides the state of Heci1 at end of boot
 	 * Setting to 0 (default) disables Heci1 and hides the device from OS */
 	uint8_t HeciEnabled;
-	/* Intel Speed Shift Technology */
-	uint8_t speed_shift_enable;
 
 	/* Enable/Disable EIST. 1b:Enabled, 0b:Disabled */
 	uint8_t eist_enable;