Crypto: Clean up psa_key_attributes_t structure definition

Define the type psa_key_attributes_t in the same way as Mbed TLS, as a
struct psa_key_attributes_s which has different definitions on the
client and server sides. This avoids needing to patch upstream
headers.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Change-Id: I240349c725facca6e353e309cccfabda289adb46