x86: kconfig: Imply ENABLE_MRC_CACHE in the platform Kconfig

Platform knows whether MRC cache is implemented, but using it can
be a choice of a specific board.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/cougarcanyon2_defconfig b/configs/cougarcanyon2_defconfig
index 84856aa..ea56e58 100644
--- a/configs/cougarcanyon2_defconfig
+++ b/configs/cougarcanyon2_defconfig
@@ -2,6 +2,7 @@
 CONFIG_VENDOR_INTEL=y
 CONFIG_DEFAULT_DEVICE_TREE="cougarcanyon2"
 CONFIG_TARGET_COUGARCANYON2=y
+# CONFIG_ENABLE_MRC_CACHE is not set
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_CONSOLE_MUX=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y