Some of the bits and fields used for mailboxes communicating with the
PSP are common across all mailbox implementations (SEV, TEE, etc).

Move these bits into the common `linux/psp.h` so they don't need to
be re-defined for each implementation.
Acked-by: Rijo Thomas <Rijo-john.Thomas@amd.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>