feat(rme): add dummy platform token to RMMD

Add a dummy platform token to RMMD and return it on request. The
platform token is requested with an SMC with the following parameters:
    * Fid (0xC40001B3).
    * Platform token PA (the platform token is copied at this address by
      the monitor). The challenge object needs to be passed by
      the caller in this buffer.
    * Platform token len.
    * Challenge object len.

When calling the SMC, the platform token buffer received by EL3 contains
the challenge object. It is not used on the FVP and is only printed to
the log.

Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>
Signed-off-by: Subhasish Ghosh <subhasish.ghosh@arm.com>
Change-Id: I8b2f1d54426c04e76d7a3baa6b0fbc40b0116348