arm: move GIC SGI kicking into separate function

arm: move GIC SGI kicking into separate function

Currently we unconditionally send SGIs to all cores on SMP bringup.

Those SGIs (software generated interrupts) are to push a secondary core
through a gate in the Xen bring up code to filter the right CPU. This gate is
necessary on platforms which do not allow us to wake up a specific secondary
processor and will trap all but the CPU we are trying to wake up.

With PSCI we can explicitly specify the core to startup, so we don't need the
kick here because the CPU will fall straight through Xen's gate.

So we move the GIC kick into a function and call it explicitly from the
platforms that need it. This gets us get rid of the empty cpu_up() platform
functions in ARM32 and the comment in there.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- explain more about the Xen gate in the commit message ]