Name UART based on address, not on the name in the datasheet

[TG9541]

There are many names for STM8(S) UARTS: UART1, UART2, UART3, UART4, LINUART. Sometimes the same silicon has two names for a UART. If you use a sub-set of the features (plain old UART) the only thing that really matters is addresses.

Things get a lot easier and more transparent when UART1 is consistently used for the first UART address set, and UART2 for the second. It doesn't matter if the UART at the second address set is the only UART (as in STM8S Medium Density, e.g. STM8S105K6T6C).

This should solve the problem in #292 (see comment) and also make applications that use a UART more portable across devices.