Removed the old dynamically allocated GDMA channel approach.
It proved too unreliable as we couldn't not ensure consumers of the mbedtls
would properly free the channels after use.
Replaced by a single shared GDMA channel for AES and SHA, which won't be
released unless user specifically calls API for releasing it.
DMA process didnt wait for idle before returning.
This meant that there was a potential for the dma
descriptors which were on the stack to be reclaimed
before the DMA operation finished.
