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/asmmacro-64.h b/include/asm-mips/asmmacro-64.h
index bbed355..559c355 100644
--- a/include/asm-mips/asmmacro-64.h
+++ b/include/asm-mips/asmmacro-64.h
@@ -8,7 +8,7 @@
 #ifndef _ASM_ASMMACRO_64_H
 #define _ASM_ASMMACRO_64_H
 
-#include <asm/offset.h>
+#include <asm/asm-offsets.h>
 #include <asm/regdef.h>
 #include <asm/fpregdef.h>
 #include <asm/mipsregs.h>