Allow P-384 and SHA-384 in RKP v3

Following requests from partners, document P-384 and SHA-384 as
officially supported signing algorithms and hash functions in the DICE
chain.

Bug: 265455904
Test: n/a -- documentation-only change
Change-Id: Id7b5eaf81be17fda9278dc7ad5f2b441931c6b83
3 files changed