mod_scmi_sys_power: Fix ret val for unsupported notifications

Section 4.4.2.3 of the SCMI v2 spec specifies that when
notifications are not supported, PROTOCOL_MESSAGES_ATTRIBUTES
must return NOT_SUPPORTED when any notification cmd is queried.

Change-Id: Ibdf0b902947bdd72903d7db9d491ff8981bf8633
Signed-off-by: Luca Vizzarro <Luca.Vizzarro@arm.com>
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>