[media] st-hva: add debug file system

This patch creates 4 static debugfs entries to dump:
- the device-related information ("st-hva/device")
- the list of registered encoders ("st-hva/encoders")
- the current values of the hva registers ("st-hva/regs")
- the information about the last closed instance ("st-hva/last")

It also creates dynamically a debugfs entry for each opened instance,
("st-hva/<instance identifier>") to dump:
- the information about the frame (format, resolution)
- the information about the stream (format, profile, level,
  resolution)
- the control parameters (bitrate mode, framerate, GOP size...)
- the potential (system, encoding...) errors
- the performance information about the encoding (HW processing
  duration, average bitrate, average framerate...)
Each time a running instance is closed, its context (including the
debug information) is saved to feed, on demand, the last closed
instance debugfs entry.

Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Jean-Christophe Trotin <jean-christophe.trotin@st.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>