gpio-rcar: Remove #ifdef CONFIG_OF around OF-specific sections

All functions and data types used by OF-specific code paths are declared
in <linux/of.h> regardless of CONFIG_OF. Replace the #ifdef CONFIG_OF
guard with a if(IS_ENABLED(CONFIG_OF)) and let the compiler optimize
the unused code away.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>