dt-bindings: mtd: jedec,spi-nor: Add atmel,at26* compatible string

Add atmel,at26* compatible string to fix below CHECK_DTB warning:

arch/arm/boot/dts/nxp/vf/vf610-twr.dtb: /soc/bus@40000000/spi@4002c000/at26df081a@0:
    failed to match any schema with compatible: ['atmel,at26df081a']

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Pratyush Yadav <pratyush@kernel.org>
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/r/20250523155258.546003-1-Frank.Li@nxp.com