clk: renesas: mstp: Convert to readl_poll_timeout_atomic()

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

As typically no retries are needed, 10 µs is a suitable timeout value.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/bce7d0bdd80c800aa150f1868b610b7d94f4cc66.1685692810.git.geert+renesas@glider.be