We need to use ffa_features() in ffa_partition_probe() to detect if
the newer FFA_PARTITION_INFO_GET_REGS API is supported in the platform
or not. To avoid unnecessary forward declaration within the file, let
us just move this ffa_features() earlier.

No funtional change.

Message-Id: <20240820-ffa_v1-2-v2-3-18c0c5f3c65e@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>