kbuild: mips use generic asm-offsets.h support

Removed obsolete stuff from arch makefile.
mips had a special rule for generating asm-offsets.h so preserved it
using an architecture specific hook in top-level Kbuild file.
Renamed .h file to asm-offsets.h

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/arch/mips/lib/memcpy.S b/arch/mips/lib/memcpy.S
index 90ee8d4..a78865f 100644
--- a/arch/mips/lib/memcpy.S
+++ b/arch/mips/lib/memcpy.S
@@ -14,7 +14,7 @@
  */
 #include <linux/config.h>
 #include <asm/asm.h>
-#include <asm/offset.h>
+#include <asm/asm-offsets.h>
 #include <asm/regdef.h>
 
 #define dst a0