powerpc/xmon: Define static functions

Currently sparse is complaining about three issues on the xmon code. Two
storage classes issues and a dereferencing a 'noderef'  pointer. These are
the warnings:

	arch/powerpc/xmon/xmon.c:2783:1: warning: symbol 'dump_log_buf' was not declared. Should it be static?
	arch/powerpc/xmon/xmon.c:2989:6: warning: symbol 'format_pte' was not declared. Should it be static?
	arch/powerpc/xmon/xmon.c:2983:30: warning: dereference of noderef expression

This patch fixes all of them, turning both functions static and
dereferencing a pointer calling rcu_dereference() instead of a
straightforward dereference.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>