omap: i2c: Fix muxing for command line enabled bus

The commit b63128e81214cc2db2995d690438055c26d213a5 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>