powerpc/fsl_rio: Use of_iomap()

Replace of_address_to_resource()+ioremap() with a call to of_iomap()
which does both of those steps.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230327223103.820229-1-robh@kernel.org