input: Add support for keyboard matrix decoding from an fdt

Matrix keyboards require a key map to be set up, and must also deal with
key ghosting.

Create a keyboard matrix management implementation which can be leveraged
by various keyboard drivers. This includes code to read the keymap from
the FDT and perform debouncing.

Signed-off-by: Bernie Thompson <bhthompson@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>