soc/intel/dnv_ns: add the Kconfig value for CPU_XTAL_HZ

Reference: Intel doc#558579 rev2.2

Change-Id: Iab5dca6eb42abc00bc7da33f640350e994f0bd02
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57945
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/soc/intel/denverton_ns/Kconfig

@@ -93,6 +93,9 @@ config CPU_BCLK_MHZ
 	int
 	default 100
 
+config CPU_XTAL_HZ
+	default 24000000
+
 config SMM_TSEG_SIZE
 	hex
 	default 0x200000