x86: mp: Allow running functions on multiple CPUs

Add a way to run a function on a selection of CPUs. This supports either
a single CPU, all CPUs, just the main CPU or just the 'APs', in Intel
terminology.

It works by writing into a mailbox and then waiting for the CPUs to notice
it, take action and indicate they are done.

When SMP is not yet enabled, this just calls the function on the main CPU.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>