stm32: Add CONFIG_DCACHE=n to stm32h7 uart API tests

As now the CONFIG_NOCACHE_MEMORY is not responsible for controlling the
data cache on STM32H7 SoC, the CONFIG_DCACHE=n must be set explicitly
to preserve previous behavior as UART driver is not using no-cache
buffers.

Considering the above comment, the CONFIG_NOCACHE_MEMORY can be safely
removed.

Signed-off-by: Lukasz Majewski <lukma@denx.de>