clk: renesas: rcar-gen3: Factor out cpg_reg_modify()

There's quite often repeated sequence of a CPG register read-modify-write,
so it seems worth factoring it out into a function -- this saves 68 bytes
of the object code (AArch64 gcc 4.8.5) and simplifies protecting all such
sequences with a spinlock in the next patch...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>