[dev] Add simple multi-instance device driver model.

Model supports class drivers capable of driving multiple device
instances independently. Implements simple pattern for device life
cycle management. Completely optional and self-contained.
[dev] Add device class APIs for block IO, I2C, and SPI.

Device class APIs are generic interfaces common to drivers of the
same class. An interface is comprised of a standard set of driver
ops and API wrappers that make use of those ops to carry out the
desired actions on the driver.
[git] Add cscope.* to .gitignore.
[kernel] Keep load percentage from underflowing.