mx6: Fix use of improper value in enable_ipu_clock

The value MXC_CCM_CCGR3_IPU1_IPU_DI0_OFFSET that was used to initialize
the CCGR3 register caused an undefined value for CG0.

Signed-off-by: Pierre Aubert <p.aubert@staubli.com>
CC: Stefano Babic <sbabic@denx.de>
Acked-by: Eric Nelson <eric.nelson@boundarydevices.com>