tests: drivers: dma test on stm32f7 requires nocache memory

Config the sram0 to be non-cachable to PASS the DMA testcases
chan_blen_transfer and loop_transfer
on the stm32f746zg and stm32f767zi nucleo boards.

The CONFIG_NOCACHE_MEMORY is useless as the memory region
gets the NOCACHE ATTRibutes for stm32H7 or stm32F7 as well.

Signed-off-by: Francois Ramu <francois.ramu@st.com>