arm-bsp/scp-firmware:Differentiate between reset and shutdown request

In Kronos, There is no differentiation between a reset and
a shutdown request. A shutdown request results in a system
reset which is an incorrect behavior.

Update the scp platform to send the shutdown request to RSS
over MHU doorbell channel.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>