dt: add of_get_child_count helper function
dt: add of_get_child_count helper function

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 <9069e6f5a2b566e2674a0ba1e2bf39c12c195fad@secretlab.ca>
Acked-by: Rob Herring <aa00819e304141d872f2e4b1c4be422ea3abbfe1@calxeda.com>
Acked-by: Stephen Warren <5ef2a23ba3aff51d1cfc8c113c1ec34b608b3b13@wwwdotorg.org>
Signed-off-by: Dong Aisheng <9173d4d6327a6bcf6e27f28380b5e57575ca23f8@linaro.org>
Signed-off-by: Linus Walleij <9cd9d802d23c0ed5e224beabf4ae4a5c478746ef@linaro.org>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas