Sometimes it needs to check if there is a subnode of given node in FDT
by given name. Introduce this helper to get the subnode if it exists.

CC: Rob Herring <robh@kernel.org>
Signed-off-by: NShannon Zhao <shannon.zhao@linaro.org>
Acked-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: NRob Herring <robh@kernel.org>
Tested-by: NJulien Grall <julien.grall@arm.com>