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 <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Acked-by: Pratyush Yadav <[e-mail]>
Signed-off-by: Pratyush Yadav <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml

···
           - pattern: "^((((micron|spansion|st),)?\
               (m25p(40|80|16|32|64|128)|\
               n25q(32b|064|128a11|128a13|256a|512a|164k)))|\
-              atmel,at25df(321a|641|081a)|\
+              atmel,at(25|26)df(321a|641|081a)|\
               everspin,mr25h(10|40|128|256)|\
               (mxicy|macronix),mx25l(4005a|1606e|6405d|8005|12805d|25635e)|\
               (mxicy|macronix),mx25u(4033|4035)|\