[i2c, doc] Improve consistency regarding device / target mode

### Description

For quite some time now, OpenTitan consistently uses the terms `host` and `device` in documentation, RTL code, specifications etc. One exception is the I2C specification where other than `host` and `device` also `target` and `target device` or `target mode` occur. Actually, in most places `target` is used. Interestingly the tests for the `target` mode are then labeled with `device`.

We should improve consistency here.

Review the I2C specification and the tests labeled for target mode, then search related documentation, RTL, and specifications for the competing host/device and target terminology. Identify the project-wide convention to apply and verify that the terminology is consistent across the affected material and test labels.

Written by the indexing model from the issue text.