i2c: begin split for am335x (bone) & dm37xx (xm)

The am335x is different enough from the dm37xx that a thin abstraction
layer needs to be created instead of a ton of #ifdefs everywhere. This
is the first step in abstracting the two SoCs.

Tested in Linaro. Next step is to rebase against Kees' latest work
and test on BBB.