firmware: arm_ffa: Use xa_insert() and check for result

While adding new partitions descriptors to the XArray the outcome of the
stores should be checked and, in particular, it has also to be ensured
that an existing entry with the same index was not already present, since
partitions IDs are expected to be unique.

Use xa_insert() instead of xa_store() since it returns -EBUSY when the
index is already in use and log an error when that happens.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20240108-ffa_fixes_6-8-v1-5-75bf7035bc50@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>