arm64: dts: imx8mm-beacon-som: Fix atmel,24c64 EEPROM compatible

Correct the EEPROM node compatible to match device tree schema (invalid
space, unknown ID) to fix dtbs_check warnings:

  arch/arm64/boot/dts/freescale/imx8mm-beacon-kit.dt.yaml: eeprom@50:
    compatible: ['microchip, at24c64d', 'atmel,24c64'] is not valid under any of the given schemas
  arch/arm64/boot/dts/freescale/imx8mm-beacon-kit.dt.yaml: eeprom@50:
    compatible:0: 'microchip, at24c64d' does not match '^[a-zA-Z][a-zA-Z0-9,+\\-._]+$'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>