dt-bindings: net: microchip: Allow nvmem-cell usage

MAC address can be provided by a nvmem-cell, thus allow referencing a
source for the address. Fixes the warning:
arch/arm/boot/dts/nxp/imx/imx6q-mba6a.dtb: ethernet@1: 'nvmem-cell-names',
 'nvmem-cells' do not match any of the regexes: 'pinctrl-[0-9]+'
 From schema: Documentation/devicetree/bindings/net/microchip,lan95xx.yaml

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20231012080033.2715241-3-alexander.stein@ew.tq-group.com