powerpc: add __NR_syscalls along with NR_syscalls

NR_syscalls macro holds the number of system call exist
in powerpc architecture. We have to change the value of
NR_syscalls, if we add or delete a system call.

One of the patch in this patch series has a script which
will generate a uapi header based on syscall.tbl file.
The syscall.tbl file contains the number of system call
information. So we have two option to update NR_syscalls
value.

1. Update NR_syscalls in asm/unistd.h manually by count-
   ing the no.of system calls. No need to update NR_sys-
   calls until we either add a new system call or delete
   existing system call.

2. We can keep this feature in above mentioned script,
   that will count the number of syscalls and keep it in
   a generated file. In this case we don't need to expli-
   citly update NR_syscalls in asm/unistd.h file.

The 2nd option will be the recommended one. For that, I
added the __NR_syscalls macro in uapi/asm/unistd.h along
with NR_syscalls asm/unistd.h. The macro __NR_syscalls
also added for making the name convention same across all
architecture. While __NR_syscalls isn't strictly part of
the uapi, having it as part of the generated header to
simplifies the implementation. We also need to enclose
this macro with #ifdef __KERNEL__ to avoid side effects.

Signed-off-by: Firoz Khan <firoz.khan@linaro.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>