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
a00d47f764