Refactor Huawei LTE to use Entity Description

[epenet]

Proposed change

Refactor Huawei LTE to use Entity Description
Also add DURATION, FREQUENCY and DATA_SIZE device classes

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[home-assistant]

Hey there @scop, @fphammerle, mind taking a look at this pull request as it has been labeled with an integration (huawei_lte) you are listed as a code owner for? Thanks!

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[scop]

Thought many times I'd do this, but never got around to it. Thanks!

Implementationwise looks good on surface, I'll take a closer look a bit later when I find time unless someone beats me to it.

[scop]

LGTM still, just one suggestion added, pre-approved with that 👍

Slightly annoying having to repeat the descriptions dict key and the entity description key, but I think we can live with that and at least it keeps things simple.