Add duration device class to uptime plugin

A duration device class (home-assistant/architecture#762) was added in 2022.5 (home-assistant/core#70543). This allows time spans to be formatted nicely.

ESPHome now uses this for the Uptime component (esphome/esphome#3928).

For consistency (and the nicer formatting), we should set the device class for the Uptime sensor here too.

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