i2c: dw: implement generic transfer functions

() For interrupt driven transfer, the driver already has
   a generic transfer function, so this simply exposes it
   for public consumption.
() The polling write is extended to support generic
   transfer.

Change-Id: I657db8a6376282c723728b7a875813d389cf7d27
Signed-off-by: Daniel Leung <daniel.leung@intel.com>