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

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

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

Cfr. commit 30ad3cf00e94f4a7 ("clk: renesas: rcar-gen3-cpg: Always use
readl()/writel()").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>