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/syscall.c b/arch/mips/kernel/syscall.c
index ae2a131..21e3e13 100644
--- a/arch/mips/kernel/syscall.c
+++ b/arch/mips/kernel/syscall.c
@@ -31,7 +31,7 @@
 #include <asm/cachectl.h>
 #include <asm/cacheflush.h>
 #include <asm/ipc.h>
-#include <asm/offset.h>
+#include <asm/asm-offsets.h>
 #include <asm/signal.h>
 #include <asm/sim.h>
 #include <asm/shmparam.h>