Fix type of cot_desc_ptr

The chain of trust description and the pointer pointing to its first
element were incompatible, thus requiring an explicit type cast for
the assignment.

- cot_desc was an array of
  const pointers to const image descriptors.

- cot_desc_ptr was a const pointer to
  (non-constant) pointers to const image descriptors.

Thus, trying to assign cot_desc to cot_desc_ptr (with no cast) would
generate the following compiler warning:

drivers/auth/tbbr/tbbr_cot.c:826:14: warning: initialization discards
  ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 REGISTER_COT(cot_desc);
              ^~~~~~~~

Change-Id: Iae62dd1bdb43fe379e3843d96461d47cc2f68a06
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>