clk: renesas: r8a7796: Obtain mode pin values from R-Car 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.

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