nvic: Change NVIC to support ARMv6-M

The differences from ARMv7-M NVIC are:
  * ARMv6-M only supports up to 32 external interrupts
   (configurable feature already). The ICTR is reserved.
  * Active Bit Register is reserved.
  * ARMv6-M supports 4 priority levels against 256 in ARMv7-M.

Signed-off-by: Julia Suvorova <jusual@mail.ru>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/include/hw/intc/armv7m_nvic.h b/include/hw/intc/armv7m_nvic.h
index 8bc2911..a472c9b 100644
--- a/include/hw/intc/armv7m_nvic.h
+++ b/include/hw/intc/armv7m_nvic.h
@@ -57,6 +57,7 @@
     VecInfo sec_vectors[NVIC_INTERNAL_VECTORS];
     /* The PRIGROUP field in AIRCR is banked */
     uint32_t prigroup[M_REG_NUM_BANKS];
+    uint8_t num_prio_bits;
 
     /* v8M NVIC_ITNS state (stored as a bool per bit) */
     bool itns[NVIC_MAX_VECTORS];