dt-bindings: clock: stm32h7: rename USART{7,8}_CK to UART{7,8}_CK

dt-bindings: clock: stm32h7: rename USART{7,8}_CK to UART{7,8}_CK

As stated in the reference manual RM0433, the STM32H743 MCU has
USART1/2/3/6, UART4/5/7/8, and LPUART1. The patches make all the clock
macros for the serial ports consistent with the documentation.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250427074404.3278732-5-dario.binacchi@amarulasolutions.com
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>