powerpc/5200: Fix size to request_mem_region() call

The Bestcomm driver requests a memory region larger than the one
described in the device tree. This is due to an extra undocumented field
in the bestcomm register structure. This hasn't been a problem up to
now, but there is a patch pending to make the DT platform_bus support
code use platform_device_add() which tightens the rules and provides
extra checks for drivers to stay within the specified register regions.

Alternately, I could have removed the extra field from the structure,
but I'm not sure if it is still needed for resume to work. Better be
safe and leave it in.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Anatolij Gustschin <agust@denx.de>