of: add helper for getting endpoint node of specific identifiers

When there are multiple ports or multiple endpoints in a port, they have to be
distinguished by the value of reg property. It is common. The drivers can get
the specific endpoint in the specific port via this function. Now the drivers
have to implement this code in themselves or have to force the order of dt nodes
to get the right node.

Signed-off-by: Hyungwon Hwang <human.hwang@samsung.com>
Acked-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>