V4L/DVB (5362): Dvb-pll: add code for doing tuner initialization

Some tuners need or benefit from initialization, to change certain
settings from their power on default values.

Typically, tuners with TUA603x PLLs can benefit from setting the AGC TOP
value to something else.  This patch includes code to set the AGC TOP to
103 dBuV for the Thomson DTT-761x tuners, which I have experimentally
verified gives the best SNR readings, increasing SNR by about 0.19 dB
over the default value.

Other tuners can make use of this as well.  For example, the separate LG
TDVS-H06xF driver's only difference from dvb-pll is this same setting of
AGC TOP value.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>