input: Add generic keyboard input handler

Add a module which understands converting key codes (or scan codes)
to ASCII characters. It includes FIFO support and can call back to
drivers to read new characters when its FIFO is empty.

Keycode maps are provided for un-modified, shift and ctrl keys.

The plan is to use this module where such mapping is required.

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