man4: Use arch specifier of Dt consistentlyHEADmain

Use the mdoc(7) arch specifier in the document title for all remaining
manuals reflecting drivers which are only for one architecture. Most are
in architecture specific folders, but amd64 only manuals are denoted in
`man4/Makefile`.

These display at the top of the manual [0] without taking up extra lines
and are useful in any case, however I hope to use these in the Hardware
Release Notes, where architecture specificity is difficult to maintain.

[0] Example: https://man-dev.freebsd.org/boot

MFC after:		3 days
Reviewed by:		0mp
Discussed with:		emaste, imp, mhorne
Differential Revision:	https://reviews.freebsd.org/D51264

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