firmware: arm_ffa: Simplify ffa_partitions_cleanup()

On cleanup iterate the XArrays with xa_for_each() and remove the existent
entries with xa_erase(), finally destroy the XArray itself.

Remove partition_count field from drv_info since no more used anywhwere.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20240108-ffa_fixes_6-8-v1-4-75bf7035bc50@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>