mailbox: pcc: Initialize SHMEM before binding the channel with the client

The PCC channel's shared memory region must be set up before the
mailbox controller binds the channel with the client, as the binding
process may trigger client operations like startup() that may rely on
SHMEM being initialized.

Reorder the setup sequence to ensure the shared memory is ready before
binding. Initialize and map the PCC shared memory (SHMEM) prior to
calling mbox_bind_client() so that clients never observe an uninitialized
or NULL SHMEM during bind-time callbacks or early use in startup().

This makes the PCC mailbox channel bring-up order consistent and
eliminates a race between SHMEM setup and client binding.

This will be needed in channel startup to clear/acknowledge any pending
interrupts before enabling them.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: lihuisong@huawei.com
Tested-by: Adam Young <admiyo@os.amperecomputing.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>