arm: Note vendor-required status of certain MACH_TYPE values

In the cases of some boards, a MACH_TYPE number is used which is either
not registered upstream or worse (for functionality) is re-using the
number of a different (or reference) platform instead.  Make sure we
have a comment in these cases.

Cc: Albert ARIBAUD <albert.aribaud@3adev.fr>
Cc: Walter Schweizer <swwa@users.sourceforge.net>
Cc: Stefan Roese <sr@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Stefan Roese <sr@denx.de>