soc/intel: convert XTAL frequency constant to Kconfig

This converts the constant for the XTAL frequency to a Kconfig option.

Change-Id: I1382dd274eeb9cb748f94c34f5d9a83880624c18
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46018
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/soc/intel/icelake/Kconfig

@@ -127,6 +127,9 @@ config SOC_INTEL_COMMON_BLOCK_GSPI_CLOCK_MHZ
 	int
 	default 120
 
+config CPU_XTAL_HZ
+	default 38400000
+
 config DRIVERS_I2C_DESIGNWARE_CLOCK_MHZ
 	int
 	default 133