lasips2: add named input gpio to port for downstream PS2 device IRQ

lasips2: add named input gpio to port for downstream PS2 device IRQ

The named input gpio is to be connected to the IRQ output of the downstream
PS2 device and used to drive the port IRQ. Initialise the named input gpio
in lasips2_port_init() and add new lasips2_port_class_init() and
lasips2_port_realize() functions to connect the PS2 device output gpio to
the new named input gpio.

Note that the reference to lasips2_port_realize() is stored in
LASIPS2PortDeviceClass but not yet used.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Tested-by: Helge Deller <deller@gmx.de>
Acked-by: Helge Deller <deller@gmx.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20220712215251.7944-27-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>