input: serio: add support for Amstrad Delta serial keyboardport

The patch introduces a serio driver that supports a keyboard serial port found
on the Amstrad Delta videophone board.

After initializing the hardware, the driver reads its input data from a buffer
filled in by the board FIQ (Fast Interrupt Request) handler.

Standard AT keyboard driver (atkbd) will be used on top of the serio layer for
handling the E3 keyboard (called mailboard) connected to the port. Since the
device generated scancodes differ from what the atkbd expects, a custom key
code to scan code table must be loaded from userspace for the keyboard to be
useable.

Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Tony Lindgren <tony@atomide.com>