clk: renesas: rzg2l: Simplify multiplication/shift logic

"a * (1 << b)" == "a << b".

No change in generated code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/71e1cf2e30fb2d7966fc8ec6bab23eb7e24aa1c4.1645460687.git.geert+renesas@glider.be