Allow reading and writing gmux ports from userspace.

For example:

echo 4 > /sys/kernel/debug/apple_gmux/selected_port
cat /sys/kernel/debug/apple_gmux/selected_port_data | xxd -p

Will show the gmux version information (00000005 in this case)
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Orlando Chamberlain <orlandoch.dev@gmail.com>
Link: https://lore.kernel.org/r/20230303112842.3094-5-orlandoch.dev@gmail.comSigned-off-by: Hans de Goede <hdegoede@redhat.com>