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The dualroot chain of trust involves 2 root-of-trust public keys:
- The classic ROTPK.
- The platform ROTPK (a.k.a. PROTPK).

Use the cookie argument as a key ID for plat_get_rotpk_info() to return the
appropriate one. This only applies if we are using the dualroot CoT ; if using
the TBBR one, the behaviour is unchanged.

Change-Id: I400707a87ec01afd5922b68db31d652d787f79bd
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>