spidev: Completely disable the DT warning

The upstream SPI and DT maintainers are completely opposed to declaring
in Device Tree that an SPI CS line is to be managed by the spidev
driver, even though the facility is useful on a hobbyist device like a
Raspberry Pi where arbitrary devices can be attached, and the
alternative to DT declaration (spi_board_info) has been almost entirely
rendered obsolete by DT.

Continue to override their objections by disabling the warning.

See: https://github.com/raspberrypi/linux/issues/3361
     https://github.com/raspberrypi/linux/issues/1054



Signed-off-by: Phil Elwell <phil@raspberrypi.org>