Support Xilinx Zynq i2c controller.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>