powerpc: Rename exception.h to exception-64s.h

The file include/asm/exception.h contains definitions
that are specific to exception handling on 64-bit server
type processors.

This renames the file to exception-64s.h to reflect that
fact and avoid confusion.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/head_64.S

@@ -36,7 +36,6 @@
 #include <asm/thread_info.h>
 #include <asm/firmware.h>
 #include <asm/page_64.h>
-#include <asm/exception.h>
 #include <asm/irqflags.h>
 
 /* The physical memory is layed out such that the secondary processor