Improve attachment of the ehci_mv driver

Driver was modified to ensure it attaches properly to "marvell,orion-ehci"
node, which doesn't have error interrupt line defined. Neccessary
ofw_compat_data struct was added and probe procedure was altered.

Reviewed by: andrew, ian
Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4369