omap: Fix i2c platform init code for omap4

Add separate omap_i2c_add_bus functions for mach-omap1
and mach-omap2. Make the mach-omap2 init set the interrupt
dynamically to support.

This is needed to add support for omap4 in a way that
works with multi-omap builds. This will eventually get
fixed in a generic way with the omap hwmods.

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