xtensa: add __NR_syscalls along with __NR_syscall_count

xtensa: add __NR_syscalls along with __NR_syscall_count

__NR_syscall_count macro holds the number of system call
exist in xtensa architecture. We have to change the value
of __NR_syscall_count, 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 total number of system
calls information. So we have two option to update __NR-
_syscall_count value.

1. Update __NR_syscall_count in asm/unistd.h manually by
   counting the no.of system calls. No need to update __NR-
   _syscall_count until we either add a new system call or
   delete existing system call.

2. We can keep this feature it 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_syscall_count 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.
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: Max Filippov <jcmvbkbc@gmail.com>
[Max: Drop __NR_syscall_count completely, use __NR_syscalls instead]