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>