clk: renesas: rcar-gen3-cpg: Always use readl()/writel()

The R-Car Gen3 CPG/MSSR driver uses a mix of clk_readl()/clk_writel()
and readl()/writel() to access the clock registers. Settle on the
generic readl()/writel().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>