drivers: stm32: dma: fix external dcache support

Several drivers checked for the presense and availability of data cache
through Kconfig symbol. This is supported according to the current
documentation, but the symbol DCACHE masks two types of cache devices: arch
and external caches. The latter is present on some Cortex-M33 chips, like
the STM32U5xx. The external dcache is bypassed when accessing internal
SRAM and only used for external memories.

In commit a2dd2324 ("drivers: adc: stm32: dma support") the rationale
for gating dcache for adc_stm32 behind STM32H7X is only hinted at, but
reason seems to be that it was the only SOC the change was tested on. The
SOC configures DCACHE=y so it is most likely safe to swap the SOC gate for
DCACHE.

The DCACHE ifdefs are now hidden inside the shared stm32_buf_in_nocache()
implementation.

Signed-off-by: Henrik Lindblom <henrik.lindblom@vaisala.com>