mv_i2c: use structure to replace the direclty define

Add i2c_clk_enable in the cpu specific code, since previous platform it,
while new platform don't need. In the pantheon and armada100 platform,
this function is defined as NULL one.

Acked-by: Heiko Schocher <hs@denx.de>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>
Signed-off-by: Lei Wen <leiwen@marvell.com>