powerpc/usbgecko: Use of_iomap()

Replace of_get_property()+of_translate_address()+ioremap() with a call
to of_iomap() which does all those steps.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230327223109.820381-1-robh@kernel.org