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/r2300_switch.S b/arch/mips/kernel/r2300_switch.S
index f100196..0d9c4a3 100644
--- a/arch/mips/kernel/r2300_switch.S
+++ b/arch/mips/kernel/r2300_switch.S
@@ -15,7 +15,7 @@
 #include <asm/cachectl.h>
 #include <asm/fpregdef.h>
 #include <asm/mipsregs.h>
-#include <asm/offset.h>
+#include <asm/asm-offsets.h>
 #include <asm/page.h>
 #include <asm/regdef.h>
 #include <asm/stackframe.h>