input: Extend matrix-keypad device tree binding

Some matrix keypad drivers can support different numbers of rows and
columns. Add a generic binding for these.

Implementation note:

In order to implement this binding in the kernel, we will need to modify
matrix_keypad_() to look up the number of rows and cols in
the keymap. Perhaps this could be done by passing 0 for these parameters?
Many of the parameters can already be set to NULL. Ick.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>