ARM: plat-omap: remove arch specific special handling for ioremap

A generic version should replace this later.

As io.c has become nearly empty, omap_init_consistent_dma_size() is moved
into common.c so that io.c can be removed entirely.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Tested-by: Kevin Hilman <khilman@ti.com>