mb/emulation/qemu-i440fx|q35: Fix stack size

Current implementation works by luck as DCACHE area is actually RAM and
stack can grow and use that RAM outside of the area.

* Set DCACHE_BSP_STACK_SIZE to 0x4000.
* Add an assert to make sure it is set to a sane value on all platforms.

Change-Id: I71f9d74d89e4129cdc4a850acc4fc1ac90e5f628
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/29611
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/emulation/qemu-i440fx/Kconfig

@@ -41,4 +41,8 @@ config C_ENV_BOOTBLOCK_SIZE
 	hex
 	default 0x4000
 
+config DCACHE_BSP_STACK_SIZE
+	hex
+	default 0x4000
+
 endif # BOARD_EMULATION_QEMU_X86_I440FX