firmware: arm_ffa: Fix memory leak by freeing notifier callback node

Commit e0573444 ("firmware: arm_ffa: Add interfaces to request
notification callbacks") adds support for notifier callbacks by allocating
and inserting a callback node into a hashtable during registration of
notifiers. However, during unregistration, the code only removes the
node from the hashtable without freeing the associated memory, resulting
in a memory leak.

Resolve the memory leak issue by ensuring the allocated notifier callback
node is properly freed after it is removed from the hashtable entry.

Fixes: e0573444 ("firmware: arm_ffa: Add interfaces to request notification callbacks")
Message-Id: <20250528-ffa_notif_fix-v1-1-5ed7bc7f8437@arm.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>