spi_slave: add valid check for DMA buffers

The DMA cannot receive data correctly when the buffer address is not
WORD aligned. Currently we only check whether the buffer is in the DRAM
region.

The DMA always write in WORDs, so the length arguments should also be
multiples of 32 bits.

A check is added to see whether the buffer is WORD aligned and has valid
length.

components/driver/include/driver/spi_slave.h

@@ -73,7 +73,10 @@ struct spi_slave_transaction_t {
     size_t length;                  ///< Total data length, in bits
     size_t trans_len;               ///< Transaction data length, in bits
     const void *tx_buffer;          ///< Pointer to transmit buffer, or NULL for no MOSI phase
-    void *rx_buffer;                ///< Pointer to receive buffer, or NULL for no MISO phase
+    void *rx_buffer;                /**< Pointer to receive buffer, or NULL for no MISO phase.
+                                     * When the DMA is anabled, must start at WORD boundary (``rx_buffer%4==0``),
+                                     * and has length of a multiple of 4 bytes.
+                                     */
     void *user;                     ///< User-defined variable. Can be used to store eg transaction ID.
 };