PM / devfreq: exynos-bus: Add registration of interconnect child device

This patch adds registration of a child platform device for the exynos
interconnect driver. It is assumed that the interconnect provider will
only be needed when #interconnect-cells property is present in the bus
DT node, hence the child device will be created only when such a property
is present.

Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>