media: platform: exynos4-is: Use of_get_available_child_by_name()

Simplify fimc_md_is_isp_available() by using
of_get_available_child_by_name().

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>