add Xiaomi-Gateway-3

[seuros]

Brief description of the changes

Added Xiaomi Smart Multi Mode Gateway 3 , but has lot of othen generic vendors since it build by tuya i think.

Credit to @syssi for the existing implementation.

Type of changes

• New device
• Update existing device
• Removing a device
• General cleanup
• Other

Checklist:

• There are no passwords or secrets references in any examples.
  The only exceptions are !secret wifi_ssid and !secret wifi_password.
• The wifi or ethernet block has no static / manual ip address specified.
• The first configuration provided should be hardware definitions only.
  A more involved example can be provided in a separate configuration block.

[netlify]

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