crypto: ccp - Validate that platform access mailbox registers are declared

Some platforms might support platform access doorbell but not mailbox.
Add an extra guard to ensure this doesn't cause accesses to wrong ranges
if a consumer calls psp_send_platform_access_msg().

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>