timer: orion-timer: Fix problem with early static variable

We've noticed that at least one Kirkwood board (Pogo v4) has problems
with the new orion DM timer implementation. Debugging revealed that this
issue is related with the static variable "early_init_done" which does
not work correctly before relocation in all cases.

This patch removes this static variable and replaces it's functionality
via a function that detects if the timer is already initialized.

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