Adapt mtu range to support Linux loopback max MTU of 65536

[precla]

Change Scope

• Extend the range of the allowed MTU to 65536 instead of max 65535
• Changes the type from uint16 to uint32 and adds range restriction

Platform Implementations

• Linux loopback MTU value: https://elixir.bootlin.com/linux/latest/source/drivers/net/loopback.c#L203

Other

See discussion in https://github.com/openconfig/public/issues/129

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[OpenConfigBot]

Major YANG version changes in commit ffd12d16635d1f453d5581a9f5d236fcca4344e7:

[OpenConfigBot]

Compatibility Report for commit ffd12d16635d1f453d5581a9f5d236fcca4344e7: ⛔ pyangbind@1b7ee10

[dplore]

Will review on Oct 17 OC Operators call