soc/intel/cannonlake: Increase stack size from 4KiB to 8KiB

Backtracking stack used BEFORE each function call:
1. cbfs_boot_locate(&file_desc, "vbt.bin", NULL): 4104 (stack overrun)
2. locate_vbt: 4068
3. vbt_get: 4036
4. platforms_fsp_silicon_params_cb: 3924
5. do_silicon_init(&fsps_hdr): 3684 (3684-1092=2592 due to fsps)
6. fsp_silicon_init: 1092

Increase the stack size from 4kiB to 8kiB to prevent stack overrun.

TEST=No stack overrun is observed and it boots to OS properly.

Change-Id: I7e458b4489cea32449f197621ec81009ea7dd0bd
Signed-off-by: John Zhao <john.zhao@intel.com>
Reviewed-on: https://review.coreboot.org/21977
Reviewed-by: Lijian Zhao <lijian.zhao@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/intel/cannonlake/Kconfig

@@ -154,4 +154,8 @@ config C_ENV_BOOTBLOCK_SIZE
 	hex
 	default 0x4000
 
+config STACK_SIZE
+	hex
+	default 0x2000
+
 endif