Currently most code to get child count in kernel are almost same,
add a helper to implement this function for dt to use.

Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: NRob Herring <rob.herring@calxeda.com>
Acked-by: NStephen Warren <swarren@wwwdotorg.org>
Signed-off-by: NDong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>