timer: orion-timer: Only init timer once

Move the code making sure that the timer is initialized only once into
orion_timer_init(), which is called from timer_early_init() and from
orion_timer_probe(). This way the timer is not re-initialized.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Michael Walle <michael@walle.cc>
Cc: Pali Rohár <pali@kernel.org>