clocksource/drivers/owl: Improve owl_timer_init fail messages

Check the return from clocksource_mmio_init, add messages in case of
an error and in case of not having a defined clock property.

Signed-off-by: Matheus Castello <matheus@castello.eng.br>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200219004810.411190-1-matheus@castello.eng.br