nb/intel/sandybridge: Configure DCACHE_BSP_STACK_SIZE

The romstage default is to set stack guards at 0x2000 below end
of stack. The code is now overwrites some of the stack guards
so increase the stack size to a comfortable 0x2800.

Change-Id: I91f559383a987241b343e743d11291f2c100f7f5
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36884
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/northbridge/intel/sandybridge/Kconfig

@@ -76,6 +76,9 @@ config DCACHE_RAM_BASE
 	hex
 	default 0xfefe0000
 
+config DCACHE_BSP_STACK_SIZE
+	hex
+	default 0x2800
 
 if USE_NATIVE_RAMINIT