devicetree: Add DT/DT_INST_CHILD_NUM and DT/DT_INST_CHILD_NUM_STATUS_OKAY

Add a generated macro for the number of child nodes of a given node.
Add a generated macro for the number of child nodes of a given node which
children's status are "okay".

Signed-off-by: Swift Tian <swift.tian@ambiq.com>