input: gpio_kbd_matrix: add poll and scan mode support

Add a poll and scan mode for the driver. If any of these are set, the
driver does not use the GPIO interrupts to detect when the matrix has to
switch to polling mode. Instead, it keeps polling it all the time,
either by enabling all the columns and poll the rows for activity, or
just keep scanning all the time.

Poll mode is useful if the specific SoC used does not support GPIO
interrupt on all the row GPIOs at the same time, scan mode if it does
not even support selecting all the columns at the same time.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>