fsl-viu: adjust for OF based clock lookup

after device tree based clock lookup became available, the VIU driver
need no longer use the previous global "viu_clk" name, but should use
the "ipg" clock name specific to the OF node

Cc: Mauro Carvalho Chehab <m.chehab@samsung.com>
Cc: linux-media@vger.kernel.org
Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>