Fix explicit Elliptic Curve parsing (3.6 backport)

[Marandil]

Description

Fixes #4314.

• Fix pk_group_from_specified self-referencing group generator G while loading G.
• Fix pk_group_from_specified not accounting for the a = p-3 internal representation.
• Fix mbedtls_pk_parse_public_key failing to load keys with explicit curve parameters.

PR checklist

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• changelog provided
• development PR not required because: no changes to development
• TF-PSA-Crypto PR provided Fix explicit Elliptic Curve parsing TF-PSA-Crypto#705
• framework PR provided Add explicit parameter variants to ec_*_pub data files mbedtls-framework#285
• 3.6 PR provided Fix explicit Elliptic Curve parsing (3.6 backport) #10630 (this PR)
• tests provided

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