sh-pfc: Drop unused support for 1:1 physical to virtual memory mappings

Now that all PFC platform devices provide memory resources support for
registers without an associated memory resource isn't used anymore. Drop
it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>