sh: Move PMB debugfs entry initialization to later stage

... so the "sh_debugfs_root" is already available. Previously it
wasn't and in result its path was "/sys/kernel/debug/pmb" instead of
"/sys/kernel/debug/sh/pmb".

Signed-off-by: Pawel Moll <pawel.moll@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>