dts: arm: st: u0: add missing USART4 to .dtsi

[dm5tt]

Sources of information:

• address: "RM0503 Reference Manual", p. 60
• interrupts: "RM0503 Reference Manual", p. 260

Fixes: #84459

[dm5tt]

First merge request caused a warning that somehow slipped through.

Reason: Used a capital letter in the address: 40004C00 -> 40004c00

Some friendly folks told me that I could have used --amend to change my first pull request that I closed a bit too quick. Sorry for the noise.

[github-actions]

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