powerpc: Fix size calculation using resource_size()

The problem is the the calculation should be "end - start + 1" but the
plus one is missing in this calculation.

Fixes: 8626816e

 ("powerpc: add support for MPIC message register API")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>