i.MX53: fix uncorrect DMA protection settings

Within the central security unit of the i.MX53 SoC, one can set protection
level of various DMA bus master requests, distinguishing them between normal,
and secure access. Although, the access level was meant to be set correctly,
the enumeration values that denoted the kind of access were incorrect. Thereby,
until now every DMA requests was set as being secure. That bug, masked an issue
of the GPU and IPU DMA operations, which cannot being distinguished on the DMA
level, although when accessing their I/O memory mapped registers the
distinguation can be done. Therefore, as an immediate, but unsecure workaround
for the demo we allow the GPU to access secure memory via DMA.

Thanks to Andrea Barisani and Andrej Rosano from Inverse Path for discovering
this bug, as well as the hardware limitation!