dt-bindings: auxdisplay: Add Maxim MAX6958/6959

Add initial device tree documentation for Maxim MAX6958/6959.
As per reviewer's request mention the fact of absence the reset
and power enable pins, since the hardware is quite simple.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>