plat/arm: Retrieve the right ROTPK when using the dualroot CoT
plat/arm: Retrieve the right ROTPK when using the dualroot CoT

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 <4b69fcfca8da45b0c11ecda5434c5da526575d9d@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware