firmware: arm_ffa: Allow FF-A initialisation even when notification fails

FF-A notifications are optional feature in the specification. Currently
we allow to continue if the firmware reports no support for the
notifications. However, we fail to continue and complete the FF-A
driver initialisation if the notification setup fails for any reason.

Let us allow the FF-A driver to complete the initialisation even if the
notification fails to setup. We will just flag the error and continue
to provide other features in the driver.

Link: https://lore.kernel.org/r/20231024-ffa-notification-fixes-v1-1-d552c0ec260d@arm.com


Tested-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>