HMP: Do not fork-boost tasks coming from PIDs <= 2

HMP: Do not fork-boost tasks coming from PIDs <= 2

System services are generally started by init, whilst kernel threads
are started by kthreadd. We do not want to give those tasks a head
start, as this costs power for very little benefit. We do however
wish to do that for tasks which the user launches.

Further, some tasks allocate per-cpu timers directly after launch
which can lead to those tasks being always scheduled on a big CPU
when there is no computational need to do so. Not promoting services
to big CPUs on launch will prevent that unless a service allocates
their per-cpu resources after a period of intense computation, which
is not a common pattern.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Jon Medhurst <tixy@linaro.org>