mailbox: pcc: Check shared memory signature on request

ACPI 6.6 Tables 14.9 and 14.12 define the PCC shared memory
signature as the bitwise OR of 0x50434300 and the PCC subspace ID.
They also clarify that the signature is populated by the platform and
verified by OSPM. The signature is at byte offset 0 in the generic,
extended and reduced PCC shared memory layouts.

Check the signature when a client requests a PCC mailbox channel,
after mapping shared memory and before binding the mailbox client.
This keeps the check in the PCC mailbox controller instead of
duplicating it in individual clients.

Treat a signature mismatch as a warning rather than rejecting the
channel request. Making this newly added check fatal could break
existing systems whose firmware did not populate the signature
correctly even though PCC communication works. Continue to reject
shared memory that is too small to contain a signature because it
cannot be inspected safely.

Cc: Huisong Li <lihuisong@huawei.com>
Signed-off-by: Sudeep Holla <sudeep.holla@kernel.org>
Tested_by: Adam Young <admiyo@os.amperecomputing.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

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