V4L/DVB (12892): DVB-API: add support for ISDB-T and ISDB-Tsb (version 5.1)

This patch increments the DVB-API to version 5.1 in order to reflect the addition of ISDB-T and ISDB-Tsb on Linux' DVB-API.

Changes in detail:
- added a small document to describe how to use the API to tune to an ISDB-T or ISDB-Tsb channel
- added necessary fields to dtv_frontend_cache
- added a smarter clear-cache function which resets all fields of the dtv_frontend_cache
- added a TRANSMISSION_MODE_4K to fe_transmit_mode_t

Signed-off-by: Olivier Grenie <olgrenie@dibcom.fr>
Signed-off-by: Patrick Boettcher <pboettcher@dibcom.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>