iio: adc: rcar-gyroadc: use devm_platform_ioremap_resource

Avoids some local boilerplate.
Suggested by coccinelle.

CHECK   drivers/iio/adc/rcar-gyroadc.c
drivers/iio/adc/rcar-gyroadc.c:495:1-11: WARNING: Use devm_platform_ioremap_resource for priv -> regs

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: Marek Vasut <marek.vasut+renesas@gmail.com>