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/include/asm-mips/sim.h b/include/asm-mips/sim.h
index 3ccfe09..9c2af1b 100644
--- a/include/asm-mips/sim.h
+++ b/include/asm-mips/sim.h
@@ -11,7 +11,7 @@
 
 #include <linux/config.h>
 
-#include <asm/offset.h>
+#include <asm/asm-offsets.h>
 
 #define __str2(x) #x
 #define __str(x) __str2(x)