clk: renesas: Deduplicate gen3_clk_get_rate64() PLL handling

Most of the PLLx, MAIN, FIXED clock handlers are calling very similar
code, which determines parent rate and then applies multiplication and
division. The only difference is whether multiplication is fixed factor
or coming from CRx register. Deduplicate the code into a single function.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>