dt: introduce of_get_child_by_name to get child node by name

This patch introduces of_get_child_by_name function to get a child node
by its name in a given parent node.

Without this patch each driver code has to iterate the parent and do
a string compare, However having of_get_child_by_name libary function would
avoid code duplication, errors and is more convenient.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>