V4L/DVB (13601): radio-si470x: support PM functions

This patch is to support PM of the si470x i2c driver.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Acked-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>