ARM: nspire: Remove unused header file mmio.h

The Nspire boardfile platform drivers have all been converted. None
of the definitions in this header are used anymore. Remove it.

While here lets remove all the other unused headers and the file
name from in the file itself.

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