soc/intel/coffeelake: Select `X86_CLFLUSH_CAR` config

This patch selects `X86_CLFLUSH_CAR` config for running `clflush`
to invalidate the cache region based on commit 3134a81 for boot
performance improvement.

Signed-off-by: Lean Sheng Tan <sheng.tan@9elements.com>
Change-Id: Icd3d16ab2cb34dc81fc12ec139c52ecaa170528d
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73686
Reviewed-by: Subrata Banik <subratabanik@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/cannonlake/Kconfig

@@ -119,6 +119,7 @@ config CPU_SPECIFIC_OPTIONS
 	select USE_FSP_NOTIFY_PHASE_POST_PCI_ENUM
 	select USE_FSP_NOTIFY_PHASE_READY_TO_BOOT
 	select USE_FSP_NOTIFY_PHASE_END_OF_FIRMWARE
+	select X86_CLFLUSH_CAR
 
 config MAX_CPUS
 	int