soc: renesas: rcar-sysc: Use readl_poll_timeout_atomic()

Replace the open-coded polling loops by calls to the
readl_poll_timeout_atomic() helper macro.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20210125142606.1050130-1-geert+renesas@glider.be