powerpc/powernv: Add missing include to LPC code

kbuild bot spotted that one:

  arch/powerpc/platforms/powernv/opal-lpc.c: In function 'opal_lpc_init_debugfs':
>> arch/powerpc/platforms/powernv/opal-lpc.c:319:35: error: 'powerpc_debugfs_root' undeclared (first use in this function)
     root = debugfs_create_dir("lpc", powerpc_debugfs_root);
                                      ^
We neet to include the definition explicitely.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>