feat(scmi): send powerdown request to online secondary cpus

To initiate a reset or reboot, the nonsecure OS invokes the PSCI
SYSTEM_RESET function from any one core. As per the PSCI specification,
it is the responsibility of firmware to implement the system view of
the reset or reboot operation. For the platforms supported by CSS,
trigger the reset/reboot operation by sending an SGI to rest all CPUs
which are online. The CPUs respond to this interrupt by initiating its
powerdown sequence.

In addition to these changes, fix coding style issues that are not
directly related to the code being introduced in this patch.

Change-Id: I547253ee28ef7eefa78180d016893671a406bbfa
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>