gpio: Add run-time dependencies to R-Car driver

The Renesas R-Car GPIO driver is only useful on shmobile unless build
testing.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Magnus Damm <damm@opensource.se>
Cc: Alexandre Courbot <gnurou@gmail.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>