mpfs_corespi: Fix firing of stale interrupt after warm reset

After warm reset the interrupt source in the HW block is not explicitly
cleared, thus once the interrupt source is enabled the old / stale interrupt
fires immediately.

This causes a DEBUGASSERT() failure on line 808 mpfs_spi_unload_rx_fifo:
  DEBUGASSERT(nwords > 0);

arch/risc-v/src/mpfs/mpfs_corespi.c

@@ -1434,6 +1434,22 @@ static void mpfs_spi_init(struct spi_dev_s *dev)
   mpfs_spi_set_master_mode(priv, 1);
   mpfs_spi_enable(priv, 1);
 
+  /* Disable all interrupt sources */
+
+  modifyreg32(MPFS_SPI_CONTROL, MPFS_SPI_INTTXTURUN |
+                                MPFS_SPI_INTRXOVRFLOW |
+                                MPFS_SPI_INTTXDONE,
+                                0);
+
+  /* Clear all interrupt sources */
+
+  putreg32(MPFS_SPI_TXCHUNDRUN |
+           MPFS_SPI_RXCHOVRFLW |
+           MPFS_SPI_DATA_RX |
+           MPFS_SPI_TXDONE, MPFS_SPI_INT_CLEAR);
+
+  /* Then enable the interrupt */
+
   up_enable_irq(priv->plic_irq);
 }