dpaa2: fdt improve detection for dpmac/phys

'pcs-handles' are not mandatory in the device tree here so do not
enforce them. This allows us to find dpmac entries needed for phys
on the WHLE-LS1 as well.

MFC after: 3 days
Reviewed by: jceel, dsl
Differential Revision: https://reviews.freebsd.org/D42644