U85: Add support for power ramping control

Ethos-U85 supports power ramping up/down control, aimed at reducing
rapid changes in power consumption at beginning and end of intensive
operations.

Change-Id: Ifcf29dbd008d480c0d64323bedce121c53beaec4
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>