dt-bindings: mfd: Add TI-Nspire misc registers

The TI Nspire devices contain a set of registers with a seemingly
miscellaneous set of functionality. This area is known simply as the
"misc" region.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230308165557.2242-2-afd@ti.com