clocksource/drivers/asm9260: Add a check for of_clk_get

asm9260_timer_init misses a check for of_clk_get.
Add a check for it and print errors like other clocksource drivers.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191016124330.22211-1-hslester96@gmail.com