firmware: arm_ffa: Move memory allocation outside the mutex locking

The notifier callback node allocation is currently done while holding
the notify_lock mutex. While this is safe even if memory allocation may
sleep, we need to move the allocation outside the locked region in
preparation to move from using muxtes to rwlocks.

Move the memory allocation to avoid potential sleeping in atomic context
once the locks are moved from mutex to rwlocks.

Fixes: e0573444 ("firmware: arm_ffa: Add interfaces to request notification callbacks")
Message-Id: <20250528-ffa_notif_fix-v1-2-5ed7bc7f8437@arm.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>