Pad driver - read input from MD controller

Conveniently expose an API to read the status of a MD pad A.
Moreover, implement a mechanism to input arbitrary characters
from it. It goes as follow:

* Direction pad select characters. Up/Down move by one,
  Left/Right move by 5
* Start acts like Return
* A acts like Backspace
* B changes "character class": lowercase, uppercase, numbers,
  special chars. The space character is the first among special
  chars.
* C confirms letter selection

                                                        (cont.)
