aarch32: make sure irqInvalid is irq_t

The GIC platforms use irq_t for irqInvalid -- bring remaining AArch32
platforms in line with that.

irq_t is an unsigned type (when it is an integer) and enum constants are
signed. For the proofs to treat these platforms uniformly, they need to
be the same kind. They can't all be enums, because irq_t can be a more
complex type for SMP platforms.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

include/drivers/irq/am335x.h

@@ -14,9 +14,7 @@
 /* No SGIs on this platform. */
 #define NUM_SGIS 0
 
-enum irqNumbers {
-    irqInvalid = 255
-};
+irq_t irqInvalid = 255;
 
 #define CMPER_REG(base, off) ((volatile uint32_t *)((base) + (off)))
 #define CMPER_TIMER3_CLKCTRL    0x84

include/drivers/irq/bcm2836-armctrl-ic.h

@@ -122,9 +122,7 @@ volatile struct core_regs {
 #define LOCAL_TIMER_CTRL_EN_BIT 28
 #define LOCAL_TIMER_CTRL_RL_MASK MASK(28)
 
-enum irqNumbers {
-    irqInvalid = 255
-};
+irq_t irqInvalid = 255;
 
 static inline bool_t isIRQPending(void)
 {

include/drivers/irq/omap3.h

@@ -20,9 +20,7 @@
 
 #define INTCPS_SIR_IRQ_SPURIOUSIRQFLAG 0xFF0000
 
-enum irqNumbers {
-    irqInvalid = 255
-};
+irq_t irqInvalid = 255;
 
 /*
  * The struct below is used to discourage the compiler from generating literals