V4L/DVB (5947): Adding support for the MT2131 tuner.

V4L/DVB (5947): Adding support for the MT2131 tuner.

This adds support for the Microtune MT2131 tuner. 8VSB mode has been
tested but QAM support will likely require more register work. Hauppauge
have not announced any QAM devices using the MT2131 so QAM remains
undone.

For legal reasons, Microtune allowed us to write a GPL driver providing
we did not document in significant detail any of the registers. This
explains the lack of comments or defined on register names.

Signed-off-by: Steven Toth <stoth@hauppauge.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>