add dummy PV current register to attributes
add dummy PV current register to attributes

Add a line for register 777 to attributes.csv, but leave
the type blank so it isn't actually used. This way it
is visible in the file, should anyone use it as an
information source for available register numbers,
but still handled separately.

victronenergy/dbus_modbustcp,victronenergy/dbus_modbustcp,victronenergy/dbus_modbustcp,victronenergy/dbus_modbustcp