powerpc/5200: fix oops during going to standby

When going to standby mode mpc code maps the whole soc5200 node
to access warious MBAR registers. However as of_iomap uses 'reg'
property of device node, only small part of MBAR is getting mapped.
Thus pm code gets oops when trying to access high parts of MBAR.
As a way to overcome this, make mpc52xx_pm_prepare() explicitly
map whole MBAR (0xc0000).

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>