omap: Split omap_read/write functions for omap1 and omap2+

Otherwise multi-omap1 support for omap1 won't work as the cpu_class_is_omap1()
won't work until the SoC is detected.

Note that eventually these will go away, please use ioremap + read/write instead.

Signed-off-by: Tony Lindgren <tony@atomide.com>