powerpc/512x: add clock structure for Video-IN (VIU) unit

Allows using clk_get()/clk_enable()/clk_disable() for VIU
clock in the v4l2 video driver.

Signed-off-by: Hongjun Chen <hong-jun.chen@freescale.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>