clocksource: em_sti: Convert to devm_* managed helpers

Replace kzalloc, clk_get, ioremap and request_irq by their managed
counterparts to simplify error paths.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>