We always said we need analogue, digital and TDC/DTC support, so
add a type name for time converters. Meanwhile, reserve one bit
more so we may have up to 8 types -- but sure we don't want to state
what kind of physical measure does the analogue value represent.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>