clk: renesas: cpg-mssr: 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/832d29fd9aa3239ea949535309d2bdb003d40c9e.1685692810.git.geert+renesas@glider.be