[Openec] matrix_3x3.c

[Frieder Ferlemann]

Hi,

... added code for 3x3 keyboard scanning (cursors).

Does not work on real registers yet, instead uses dummy
registers for GPIOD10 and GPIOIN10.
I cannot test the code and as I probably cannot get it
right without testing so I rather committed with dummy
registers. And hope someone fixes it...

The code includes some comments. The externally visible stuff
boils down to those in matrix_3x3.h.
This might give an idea how I'd suggest a routine within the
main loop should behave:^)

Greetings,
Frieder