sh-pfc: Don't map data registers individually

All data registers are located in the same memory resource. Locate the
mapped resource at initializat time and use it directly instead of
computing a mapped address for each register. This gets rid of the
mapped_reg field of the pinmux_data_reg structure.

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