mvebu - switch to orion-timer

[Pali Rohár]

Hello Stefan! Now when U-Boot contains new orion-timer.c driver, which
Michael wrote, I think that it mvebu platform should switch to use it.
Because build process for armada boards prints deprecation warning that
new timer is not being used. Could you look at it, if it is possible to
do global switch for mach-mvebu and mach-kirkwood? I think it does not
make sense to do per-board switching as it is de-facto platform related
code.