Make it explicit that D_MEM cdevsw d_flag is to signify that the
driver is (or behaves identically to) /dev/mem. Remove the D_MEM flag
from random drivers.

Note that currently the D_MEM flag does not affect any behaviour, but
this going to change in the next commit.

Noted and reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
X-Differential revision: https://reviews.freebsd.org/D6149