arm: mvebu: timer.c: Explicitly move "init_done" var to data section

As reported by Simon Guinot, commit ade741b3
"arm: mvebu: Call timer_init early before PHY and DDR init" breaks
Kirkwood platforms. As the static variable "init_done" is not
available at that early boot time. This patch moves it to explicitly
to the data section, making it available at that time.

Signed-off-by: Stefan Roese <sr@denx.de>
Reported-by: Simon Guinot <simon.guinot@sequanux.org>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Tested-by: Simon Guinot <simon.guinot@sequanux.org>