misc/at25, dt: support probing at25 SPI EEPROM from DT

The commit d6ae0d57 introduced devicetree
binding documentation for this driver, but the driver itself does not yet
support the documented compatible entry. Fix this by adding the documented
entry to the driver.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>