omap: i2c: Fix muxing for command line enabled bus

The commit b63128e8 broke the pin muxing
for I2C busses that are enabled from the kernel command line.

Fix this by defining the board registration function omap_register_i2c_bus
in common platform code as it was before but keep the muxing in architecture
dependent files.

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