Input: ati_remote2 - add loadable keymap support

Support for loadable keymaps. The driver now supports individual
keymaps for each of the five modes (AUX1-AUX4 and PC) of the
remote. To achieve this the keymap scancode is interpreted as a
combination of the mode and actual button scancode. The original
keycode patches were done by Peter Stokes <linux@dadeos.co.uk>
but I modified it quite a lot.

Signed-off-by: Ville Syrjala <syrjala@sci.fi>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>