cmn600: Fix inappropriately-initialised global state

The CMN600 module does not account for resets of the configured
dependent clock (if any) when allocating context structures or
reconfiguring the network. The consequence of this is that resetting the
clock multiple times will eventually cause invalid register writes, a
heap allocation overflow, and lost allocated memory.

Change-Id: If5883fa63e085eb9b5f394b9c4196f68f3b8f1e5
Signed-off-by: Chris Kay <chris.kay@arm.com>