dm: timer: handle being called before dm_root is ready

dm: timer: handle being called before dm_root is ready

When used with bootstage recording, dm_timer_init may be called
surprisingly early: i.e. before dm_root is ready. To deal with
this case, we explicitly check for this condition and return
-EAGAIN to the caller (refer to drivers/timer/rockchip_timer.c
for a case where this is needed/used).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>