MPFS: Use riscv_exception_attach like the others do

Exception / fault handling goes to unexpected ISR, fix by setting the
common ISR handlers like all other RISC-V platforms do.

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

@@ -98,15 +98,9 @@ void up_irqinitialize(void)
 
   CURRENT_REGS = NULL;
 
-  /* Attach the ecall interrupt handler */
+  /* Attach the common interrupt handler */
 
-#ifndef CONFIG_ARCH_USE_S_MODE
-  irq_attach(RISCV_IRQ_ECALLM, riscv_swint, NULL);
-#endif
-
-#ifndef CONFIG_BUILD_FLAT
-  irq_attach(RISCV_IRQ_ECALLU, riscv_swint, NULL);
-#endif
+  riscv_exception_attach();
 
 #ifndef CONFIG_SUPPRESS_INTERRUPTS