drm(4): This device is MPSAFE

* And has always been since the initial import from FreeBSD 11 years ago.

* Tested with Radeon and i915 hardware for good measure.
mii: Add RealTek RTL8251 phy found on an ASUS A88XM-Plus mainboard.

Taken-From: OpenBSD
re(4): Use MPSAFE callout

The callout function was already protected by a serializer.