feat(arm): retrieve the right ROTPK for cca

The cca chain of trust involves 3 root-of-trust public keys:
- The CCA components ROTPK.
- The platform owner ROTPK (PROTPK).
- The secure world ROTPK (SWD_ROTPK).

Use the cookie argument as a key ID for plat_get_rotpk_info() to return
the appropriate one.

Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
Change-Id: Ieaae5b0bc4384dd12d0b616596596b031179044a