omap: Split i2c platform init for mach-omap1 and mach-omap2

Otherwise we cannot limit new mux code to mach-omap2.
The same signal names should eventually work for other
omaps under mach-omap2.

Note that these pins don't need to be OMAP_PIN_INPUT_PULLUP,
just OMAP_PIN_INPUT is enough.

Cc: Jarkko Nikula <jhnikula@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>