mips: rename scall64-64.S to scall64-n64.S

mips: rename scall64-64.S to scall64-n64.S

When we get nanoMIPS support we'll be introducing the p32
ABI, and there's a reasonable chance that the equivalent
p64 ABI may come along in the future. Using 'n64' now would
avoid confusion in that case where we may have 2 different
64-bit ABIs.

Suggested-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Firoz Khan <firoz.khan@linaro.org>
[paul.burton@mips.com:
  - Remove UAPI macro renaming, github code search shows at least the
    chromium project uses __NR_64_Linux & __NR_64_Linux_syscalls.]
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: y2038@lists.linaro.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Cc: arnd@arndb.de
Cc: deepa.kernel@gmail.com
Cc: marcin.juszkiewicz@linaro.org