V4L/DVB: dib7770: enable the current mirror

To improve performance on DiB7770-devices enabling the current mirror
is needed.

This patch adds an option to the dib7000p-driver to do that and it
creates a separate device-entry in dib0700-device to use those changes
on hardware which is using the DiB7770.

Signed-off-by: Olivier Grenie <olivier.grenie@dibcom.fr>
Signed-off-by: Patrick Boettcher <patrick.boettcher@dibcom.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>