gpio-rcar: use devm_ioremap_resource()

Using devm_ioremap_resource() has several advantages over devm_ioremap():
- it checks the passed resource's validity;
- it calls devm_request_mem_region() to check for the resource overlap;
- it prints an error message in case of error.

We can call devm_ioremap_resource() instead of devm_ioremap_nocache()
as ioremap() and ioremap_nocache()  are implemented identically on ARM.
Doing this saves 2 LoCs and 80 bytes (AArch64 gcc 4.8.5).

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>