ARM: psci: factor out smc command into commands/ ...

So far, the smc command was only usable when barebox also provides the
secure monitor. It's useful to have it when barebox is a PSCI consumer
as well to test whether the secure monitor works.
Factor out the code into commands/ in preparation to do so.
No functional change.

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>