V4L/DVB (12861): tda18271: add support for additional low-power standby modes

V4L/DVB (12861): tda18271: add support for additional low-power standby modes

By default, the driver enters standby mode with slave tuner output
loop thru enabled and xtal oscillator on.

Not all designs require that slave tuner output loop thru and
xtal oscillator remain active while in standby mode, so two
additional standby modes have been added:

- standby mode with xtal oscillator on (loop thru off)

- total power off

Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>