clk: renesas: rcar-gen2: Obtain mode pin values using RST driver

Obtain the values of the mode pins from the R-Car RST driver, which
relies on the presence in DT of a device node for the RST module.

Fall back to our own private copy of rcar_gen2_read_mode_pins() for
backward-compatibility with old DTs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>