dt-bindings: i2c: eeprom: Document vendor to be used and deprecated ones

The at24 driver allows to register I2C EEPROM chips using different vendor
and devices, but the I2C subsystem does not take the vendor into account
when matching using the I2C table since it only has device entries.

But when matching using an OF table, both the vendor and device has to be
taken into account so the driver defines only a set of compatible strings
using the "atmel" vendor as a generic fallback for compatible I2C devices.

Document in the Device Tree binding document that this manufacturer should
be used as the generic fallback. Also document the deprecated vendors.

Suggested-by: Wolfram Sang <wsa@the-dreams.de>
Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>