[U-Boot] [U-Boot,3/5] arm: Add PSCI shutdown function

[Alexander Graf]

> Using PSCI you can not only reset the system, you can also shut it down!
> This patch exposes a function to do exactly that to whatever code wants
> to make use of it.
> 
> Signed-off-by: Alexander Graf <agraf at suse.de>
> Reviewed-by: Simon Glass <sjg at chromium.org>

Thanks, applied to