k64f: Added SPI block write using DSPI_MasterTransferBlocking

performance improvements:
naive block writes    3.997Mbps
DSPI block writes    17.809Mbps

Author: geky

targets/TARGET_Freescale/TARGET_MCUXpresso_MCUS/TARGET_MCU_K64F/spi_api.c

@@ -130,13 +130,14 @@ int spi_master_write(spi_t *obj, int value)
 int spi_master_block_write(spi_t *obj, const char *tx_buffer, int tx_length, char *rx_buffer, int rx_length) {
     int total = (tx_length > rx_length) ? tx_length : rx_length;
 
-    for (int i = 0; i < total; i++) {
-        char out = (i < tx_length) ? tx_buffer[i] : 0xff;
-        char in = spi_master_write(obj, out);
-        if (i < rx_length) {
-            rx_buffer[i] = in;
-        }
-    }
+    DSPI_MasterTransferBlocking(spi_address[obj->spi.instance], &(dspi_transfer_t){
+        .txData = (uint8_t *)tx_buffer,
+        .rxData = (uint8_t *)rx_buffer,
+        .dataSize = total,
+        .configFlags = kDSPI_MasterCtar0 | kDSPI_MasterPcs0 | kDSPI_MasterPcsContinuous,
+    });
+
+    DSPI_ClearStatusFlags(spi_address[obj->spi.instance], kDSPI_RxFifoDrainRequestFlag | kDSPI_EndOfQueueFlag);
 
     return total;
 }