fix(rme): update sample platform attestation token

Update FVP platform attestation token to comply with RMM Beta0
specification. The changes are:
- change platform implementation id claim value from 64 to 32 bits
- change Realm Challenge
- update Hash Algorithm Identifier claim value
- add protected header
- change signing algotithm to ECDSA ES384

Change-Id: I1c5907d1a4961ce08a1408d25128de125b3f2e7f
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>