Make hme(4), i.e. the PCI-variant, MI by reading the MAC address on sytems

Make hme(4), i.e. the PCI-variant, MI by reading the MAC address on sytems
without Open Firmware directly instead of using OF_getetheraddr(). This is
a bit painful though, as the MAC address is contained in the NA field of
the VPD of the EBus bridge, which is is another function of the same chip.
To make it worse, the VPD of the EBus bridge can't be accessed via the PCI
capability pointer but has to be digged out from the Boot PROM and has a
non-standard format.
The PCI VPD struct and macros used here should be part of the FreeBSD PCI
code nevertheless.

Approved by:	tmm
Based on:	NetBSD
Tested with:	Sun X1032A (hme(4)-isp(4)-combo card) on alpha and i386