ARM: shmobile: r8a7740: Remove legacy machine_desc.map_io() callback

Commit 37201ba5c99d0be8 ("ARM: shmobile: r8a7740: Migrate to generic l2c
OF initialization") removed the last user of the legacy "IOMEM()" macro
on r8a7740-based systems. Hence there's no longer a need to set up a
transparent mapping of system I/O registers. Remove the mapping and the
legacy machine_desc.map_io() callback.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>