stm32mp: stm32prog: replace alias by serial device sequence number

The command "stm32prog serial <dev>" can directly use the device sequence
number of serial uclass as this sequence number is egual to alias when it
exist; this assumption simplify the code and avoid access to gd->fdt_blob
and the device tree parsing.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>