power_domain: Relax log group when power transition is denied

by the driver

The deny API is intentionally provided to refuse transitions
in known circumstances, hence a warning group log fits better
than an error.

Change-Id: Ibd83840e750c497d9383e00680afb090305921fa
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>