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/kernel/scall32-o32.S b/arch/mips/kernel/scall32-o32.S
index 344f2e2..17b5030 100644
--- a/arch/mips/kernel/scall32-o32.S
+++ b/arch/mips/kernel/scall32-o32.S
@@ -19,7 +19,7 @@
 #include <asm/thread_info.h>
 #include <asm/unistd.h>
 #include <asm/war.h>
-#include <asm/offset.h>
+#include <asm/asm-offsets.h>
 
 /* Highest syscall used of any syscall flavour */
 #define MAX_SYSCALL_NO	__NR_O32_Linux + __NR_O32_Linux_syscalls