dt-bindings: net: fman: remove ptp-timer from required list

IEEE1588(ptp) is optional feature for network. Remove it from required
list to fix below CHECK_DTBS warning.
arch/arm64/boot/dts/freescale/fsl-ls1043a-qds.dtb: ethernet@f0000: 'ptp-timer' is a required property

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>