N900 legacy user space apps need the board name in
/proc/cpuinfo to work properly for the Hardware entry.

For other boards this should not be an issues and they
can use the generic Hardware entry.

Let's fix the issue by adding a custom DT_MACHINE_START
for n900.
Tested-by: NPali Rohár <pali.rohar@gmail.com>
Acked-by: NPavel Machek <pavel@ucw.cz>
Signed-off-by: NTony Lindgren <tony@atomide.com>