Add the Raspberry Pi BSC (I2C compliant) controller driver.

Reviewed by:	rpaulo
Approved by:	adrian (mentor)
Move CAM_UNQUEUED_INDEX setting to the last moment and under the periph lock.
This fixes race condition with cam_periph_ccbwait(), causing use-after-free.
Enable the build of OFW I2C bus for FDT systems.

Approved by:	adrian (mentor)