ARM: OMAP: Add method to register additional I2C busses on the command line

This patch extends command line option "i2c_bus=bus_id,clkrate" so that
it allow to register additional I2C busses that are not registered with
omap_register_i2c_bus from board initialization code.

Purpose of this is to register additional board busses which are routed
to external connectors only without any on board I2C devices.

Signed-off-by: Jarkko Nikula <jarkko.nikula@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>