wii: Add support for GameCube controller sockets.

A new driver is introduced for the Serial Interface that exposes the four
GameCube controller sockets as uhid(4) devices. The report format and HID
usages of these devices attempts to mimic the official USB GameCube
controller adapter.

wii$ usbhidctl -f /dev/uhid0 -r
Report descriptor:
Collection page=Generic_Desktop usage=Game_Pad
Input size=1 count=1 page=Button usage=Button_3, logical range 0..1
Input size=1 count=1 page=Button usage=Button_1, logical range 0..1
Input size=1 count=1 page=Button usage=Button_2, logical range 0..1
Input size=1 count=1 page=Button usage=Button_4, logical range 0..1
Input size=1 count=1 page=Button usage=Button_11, logical range 0..1
Input size=1 count=1 page=Button usage=Button_12, logical range 0..1
Input size=1 count=1 page=Button usage=Button_10, logical range 0..1
Input size=1 count=1 page=Button usage=Button_9, logical range 0..1
Input size=1 count=1 page=Button usage=Button_8, logical range 0..1
Input size=1 count=1 page=Button usage=Button_6, logical range 0..1
Input size=1 count=1 page=Button usage=Button_7, logical range 0..1
Input size=1 count=1 page=Button usage=Button_5, logical range 0..1
Collection page=Generic_Desktop usage=Pointer
Input size=8 count=1 page=Generic_Desktop usage=X, logical range 0..255
Input size=8 count=1 page=Generic_Desktop usage=Y, logical range 0..255
End collection
Collection page=Generic_Desktop usage=Pointer
Input size=8 count=1 page=Generic_Desktop usage=Z, logical range 0..255
Input size=8 count=1 page=Generic_Desktop usage=Rz, logical range 0..255
End collection
Input size=8 count=1 page=Generic_Desktop usage=Rx, logical range 0..255
Input size=8 count=1 page=Generic_Desktop usage=Ry, logical range 0..255
Total input size 9 bytes
Total output size 0 bytes
Total feature size 0 bytes