gpio_matrix: doubleshot keyboard matrix debounce issue workaround

This doubleshot only fix is to prevent accidental double key press
events to escape into the input event sync mechanism.
On doubleshot keyboard some rare events were generated that were
not produced by actual keypress, but one keypress produced two
events instead. Measuring a minimum time between the pressing of
the same keycode and dropping events that would be too close
fixes the issue and does not affect the functionality in any
negative way.

Change-Id: Ib500379e0ff2b6f3a58ab1311adceca6dbe19a9c
Rebase to Linux 3.0.70