firmware: arm_ffa: Fix Endpoint Memory Access Descriptor offset calculation

firmware: arm_ffa: Fix Endpoint Memory Access Descriptor offset calculation

Use the descriptor's `ep_mem_offset` to calculate the start of the endpoint
memory access array and to comply with the FF-A spec instead of defaulting
to `sizeof(struct ffa_mem_region)`.
This requires moving `ffa_mem_region_additional_setup()` earlier in the setup
flow.
Also, add sanity checks to ensure the calculated descriptor offsets do not
exceed `max_fragsize`.

Fixes: 113580530ee7 ("firmware: arm_ffa: Update memory descriptor to support v1.1 format")
Reviewed-by: Sudeep Holla <sudeep.holla@kernel.org>
Signed-off-by: Mostafa Saleh <smostafa@google.com>
Signed-off-by: Sebastian Ene <sebastianene@google.com>
Link: https://patch.msgid.link/20260702103848.1647249-3-sebastianene@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>