soc: renesas: Provide dummy rcar_rst_read_mode_pins() for compile-testing

If the R-Car RST driver is not included, compile-testing R-Car clock
drivers fails with a link error:

    undefined reference to `rcar_rst_read_mode_pins'

To fix this, provide a dummy version.  Use the exact same test logic as
in drivers/soc/renesas/Makefile, as there is no Kconfig symbol (yet) to
control compilation of the R-Car RST driver.

Fixes: 527c02f6

 ("soc: renesas: Add R-Car RST driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>