pinctrl: renesas: rzv2m: Use devm_clk_get_enabled() helper

Simplify clock handling by using the devm_clk_get_enabled() helper,
instead of open-coding the same operations.  Move the clock pointer from
the driver-private data to a local variable, as it is not needed outside
the .probe() callback.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/cca0b1795fd0335401bdf2be393ab84445e04095.1688396836.git.geert+renesas@glider.be