Make MAX_PHYSICAL_CPUS invisible on non-AMD boards

It's only used on AMD based boards. Hence drop it, so we don't
accidently start using it by mistake instead of MAX_CPUS

Change-Id: Id8f522f24283129874d56e70bd00df92abe9c3cf
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/1325
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/cpu/Kconfig

@@ -23,6 +23,7 @@ config DCACHE_RAM_GLOBAL_VAR_SIZE
 # yet be dropped completely.
 config MAX_PHYSICAL_CPUS
 	int
+	depends on CPU_AMD_MODEL_10XXX || CPU_AMD_MODEL_FXX || CPU_AMD_AGESA
 	default 1
 
 config SMP