net: marvell: prestera: Avoid unnecessary DT lookups

This driver fist makes an expensive DT lookup to retrieve its DT node
(this is a PCI driver) in order to later search for the
base-mac-provider property. This property has no reality upstream and
this code should not have been accepted like this in the first
place. Instead, there is a proper nvmem interface that should be
used. Let's avoid these extra lookups and rely on the nvmem internal
logic.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>