dpaa2-mac: bail if the dpmacs fwnode is not found

The parent pointer node handler must be declared with a NULL
initializer. Before using it, a check must be performed to make
sure that a valid address has been assigned to it.

Signed-off-by: Robert-Ionut Alexa <robert-ionut.alexa@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>