spi: Added default spi_master_block_write implementation to stm targets

There is an easy default implementation of spi_master_block_write that
just calls spi_master_write in a loop, so the default implementation
of spi_master_block_write has been added to all targets.

Author: sg-

targets/TARGET_STM/stm_spi_api.c

@@ -367,6 +367,21 @@ 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;
+        }
+    }
+
+    return total;
+}
+
 int spi_slave_receive(spi_t *obj)
 {
     return ((ssp_readable(obj) && !ssp_busy(obj)) ? 1 : 0);