Define RAMTOP for x86 only

This Kconfig is deprecated, new platforms need to locate
ramstage stack in CBMEM instead.

Change-Id: I20ece297302321337cc2ce17fdef0c55242a4fc3
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/15189
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/Kconfig

@@ -469,11 +469,6 @@ config TPM
 
 	  If unsure, say N.
 
-config RAMTOP
-	hex
-	default 0x200000
-	depends on ARCH_X86
-
 config HEAP_SIZE
 	hex
 	default 0x4000