soc: renesas: rmobile-sysc: Convert to readl_poll_timeout_atomic()

Use readl_poll_timeout_atomic() instead of open-coding the same
operation.

  1. rmobile_pd_power_down(): as typically less than 20 retries are
     needed, PSTR_RETRIES (100) µs is a suitable timeout value.

  2. __rmobile_pd_power_up(): the old method of first polling some
     cycles with a 1 µs delay, followed by more polling cycles without
     any delay didn't make much sense, as the latter was insignificant
     compared to the former.  Furthermore, typically no retries are
     needed.  Hence just retain the polling with delay.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/ae4bf03ab8fd5a557c683086958d6764babc0723.1685692810.git.geert+renesas@glider.be