i2c: designware: add an implement i2c protos

Include the i2c header and change the non confirming
functions to do so.

Cc: Heiko Schocher <hs@denx.de>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Acked-by: Heiko Schocher <hs@denx.de>
[trini: Fix i2c_get_bus_num prototype]
Signed-off-by: Tom Rini <trini@ti.com>