hvc_muen: Use sinfo API to determine console channel

The module requests a channel with name 'virtual_console' via the sinfo
API.
Define channel_info struct in sinfo API

Use this struct as argument in the muen_get_channel_info function.
Add muen_for_each_channel function to sinfo API

The muen_for_each_channel function can be used to iterate over all
available channels. Channel information and the optional data argument
are passed to a specified callback function.
muen-sinfo: Implement muen_for_each_channel function
muen-sinfo: Log channel information on init
Fix sparse warnings