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/stackframe.h b/include/asm-mips/stackframe.h
index fb42f99..7b5e646 100644
--- a/include/asm-mips/stackframe.h
+++ b/include/asm-mips/stackframe.h
@@ -15,7 +15,7 @@
 
 #include <asm/asm.h>
 #include <asm/mipsregs.h>
-#include <asm/offset.h>
+#include <asm/asm-offsets.h>
 
 		.macro	SAVE_AT
 		.set	push