V4L/DVB (5313): Added a config entry and a gpio function pointer to tuner struct

These entries mainly are to support configurations of the tda827x
silicon tuner with a preamplifier.
The values can be set throgh the attach inform or through
the extended TUNER_SET_TYPE_ADDR client call. The function pointer
will only be updated if the parameter is not NULL.
Since a typecast is necessary to set the pointer, i added a typedef for
this pointer (tuner_gpio_func_t) in tuner.h

Signed-off-by: Hartmut Hackmann <hartmut.hackmann@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>