[sw, dif_uart] Add "last" enum element
[sw, dif_uart] Add "last" enum element

Knowing how many elements are in the enum can be useful to write more
robust iterators over the enum.

Signed-off-by: Silvestrs Timofejevs <5a6466a3751cccdb6ee413d1e87698ed8baaba81@lowrisc.org>

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