i2c-designware: Push all register reads/writes into the core code.

Move all register manipulation code into the core, also move register
offset definitions to i2c-designware-core.c since the bus specific
portions of the driver no longer need/use them.

Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>