dt-bindings: at24: add new compatible

arch/arm/boot/dts/at91-dvk_som60.dt.yaml uses the compatible string
'giantec,gt24c32a' for an at24 EEPROM with a fallback to 'atmel,24c32'.

Add this model as a special case to the binding document.

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Rob Herring <robh@kernel.org>