firmware: arm_ffa: Fix Endpoint Memory Access Descriptor offset calculation

In the Linux kernel, the following vulnerability has been resolved: 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`.