[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: David S. Miller <davem@davemloft.net>
diff --git a/arch/sparc64/Kconfig b/arch/sparc64/Kconfig
index d1e2fc5..648047a 100644
--- a/arch/sparc64/Kconfig
+++ b/arch/sparc64/Kconfig
@@ -194,6 +194,9 @@
 
 endmenu
 
+config ARCH_SELECT_MEMORY_MODEL
+	def_bool y
+
 config ARCH_SPARSEMEM_ENABLE
 	def_bool y