OMAP: I2C: split device registration and convert OMAP2+ to omap_device

Split the OMAP1 and OMAP2+ platform_device build and register code.
Convert the OMAP2+ variant to use omap_device.

This patch was developed in collaboration with Rajendra Nayak
<rnayak@ti.com>.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>