lasips2: move mouse port initialisation to new lasips2_mouse_port_init() function

lasips2: move mouse port initialisation to new lasips2_mouse_port_init() function

Move the initialisation of the mouse port from lasips2_init() to
a new lasips2_mouse_port_init() function which will be invoked using
object_initialize_child() during the LASIPS2 device init.

Update LASIPS2State so that it now holds the new LASIPS2MousePort child object and
ensure that it is realised in lasips2_realize().

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: <20220712215251.7944-21-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>