[opentitanlib] Single source of truth for UART baudrates

### Description

See https://github.com/lowRISC/opentitan/pull/20706#pullrequestreview-1794578270

We set the initial baud rate for UARTs in OpenTitanLib using constants in the source. These bauds are also specified in the JSON configurations for each target.

There should only really be a single source of truth for these, likely the JSON config.

Compare the UART baudrate constants in OpenTitanLib with the baudrates in each target's JSON configuration. Start by locating both definitions and tracing how the initial UART rate is selected. Done means the baudrate has one authoritative source, the other copy is removed, and all targets still receive the intended initial rate.

Written by the indexing model from the issue text.