boards/kakute-f7-mini-v3: map default RAM to SRAM1

boards/kakute-f7-mini-v3/memory.x

@@ -1,5 +1,20 @@
 MEMORY
 {
   FLASH : ORIGIN = 0x08000000, LENGTH = 512K
-  RAM : ORIGIN = 0x20000000, LENGTH = 256K
+
+  /*
+   * STM32F722 RAM totals 256 KiB:
+   * - DTCM:  64 KiB at 0x2000_0000; inaccessible to DMA1 and DMA2.
+   * - SRAM1: 176 KiB at 0x2001_0000; contiguous and DMA2-accessible.
+   * - SRAM2:  16 KiB at 0x2004_C000; a separate bank after an address gap.
+   *
+  * Why use SRAM1 as the only default RAM region: the MPU6000 SPI DMA buffers
+  * and USART1 TX DMA frames are ordinary RTIC task-local/static allocations.
+  * Mapping `RAM` to DTCM would let the linker place those buffers where DMA2
+  * cannot read or write them which would lead transfer faults or corrupted data.
+  *
+  * Therefore all default allocations use SRAM1 to make DMA reachability the
+  * safe default rather than a per-buffer convention.
+   */
+  RAM : ORIGIN = 0x20010000, LENGTH = 176K
 }