ARM: OMAP3: Make I2C bus 2 configurable for BeagleBoard

ARM: OMAP3: Make I2C bus 2 configurable for BeagleBoard

I2C2 at BeagleBoard is connected to expansion connector, i.e. unused if
nothing is connected to this connector. As internal OMAP3 pull up resistors
are not strong enough, enabled but unused I2C2 bus results in error messages
(e.g. I2C timeouts). I2C2 should be enabled only if something is connected to
I2C2 at board's expansion connector and this extension has additional pull up
resistors for I2C2 bus.

- Add configuration option for this
- Use configuration option in board-omap3beagle
- While being there, add OMAP3 to OMAP I2C help text

Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>