gpio: rcar: Improve clock error handling and reporting

If the Renesas R-Car GPIO driver cannot find a functional clock, it
prints a warning, .e.g.

	gpio_rcar ffc40000.gpio: unable to get clock

and continues, as the clock is optional, depending on the SoC type.
This warning may confuse users.

To fix this, add a flag to indicate that the clock is mandatory or
optional:
  - If the clock is mandatory (on R-Car Gen2 and Gen3), a missing clock
    is now treated as a fatal error,
  - If the clock is optional (on R-Car Gen1), the warning is no longer
    printed.

Suggested-by: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>