ARM: mach-nspire: Rework support and directory structure

Having a platform need a mach-* directory should be seen as a negative,
it means the platform needs special non-standard handling. ARM64 support
does not allow mach-* directories at all. While we may not get to that
given all the non-standard architectures we support, we should still try
to get as close as we can and reduce the number of mach directories.

The mach-nspire/ directory and files, provides just one "feature":
having the kernel print the machine name if the DTB does not also contain
a "model" string (which they always do). To reduce the number of mach-*
directories let's do without that feature and remove this directory.

NOTE: The default l2c_aux_mask is now ~0 but these devices never have
this type of cache controller so this is safe.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>