Change the default IRQs.

* Reserve IRQ 0 to be used as a "non-maskable interrupt" (NMI) in the
   future.

 * Use IRQ 2 for the LiteX. This matches the standard mor1k config which
   connects the UART to IRQ 2.

This change is needed for Linux running on LiteX as it gets grumpy with
using IRQ 0 for anything other other than an NMI.

litex/soc/integration/soc_core.py

@@ -36,8 +36,9 @@ class SoCCore(Module):
         "leds":           6,  # user
     }
     interrupt_map = {
-        "uart":   0,
-        "timer0": 1,
+        "nmi":    0, # Reserve zero for "non-maskable interrupt"
+        "timer0": 1, # LiteX Timer
+        "uart":   2, # LiteX UART (IRQ 2 for UART matches mor1k standard config).
     }
     mem_map = {
         "rom":      0x00000000,  # (default shadow @0x80000000)