[SPARC64]: Set ARCH_SELECT_MEMORY_MODEL

Otherwise the build breaks with EXPERIMENTAL disabled
because SPARSEMEM will not get selected properly.  See
mm/Kconfig for how that works.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc64/Kconfig

@@ -194,6 +194,9 @@ endchoice
 
 endmenu
 
+config ARCH_SELECT_MEMORY_MODEL
+	def_bool y
+
 config ARCH_SPARSEMEM_ENABLE
 	def_bool y