cputlb: Make tlb_n_entries private to cputlb.c

cputlb: Make tlb_n_entries private to cputlb.c

There are no users of this function outside cputlb.c,
and its interface will change in the next patch.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>