spi: Add driver_override SPI device attribute

spi: Add driver_override SPI device attribute

This attribute works the same was as the identically named attribute
for PCI, AMBA, and platform devices.  For reference, see:

commit 169487d3392c ("ARM: 8256/1: driver coamba: add device binding
path 'driver_override'")
commit 84e297234a4b ("driver core: platform: add device binding path
'driver_override'")
commit dbb735495ebe ("PCI: Introduce new device binding path using
pci_dev.driver_override")

If the name of a driver is written to this attribute, then the device
will bind to the named driver and only the named driver.

The device will bind to the driver even if the driver does not list the
device in its id table.  This behavior is different than the driver's
bind attribute, which only allows binding to devices that are listed as
supported by the driver.

It can be used to bind a generic driver, like spidev, to a device.

Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Tested-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>