ArmPkg/ArmGic: Fix maximum number of interrupts in GICv3

Bugzilla: 3415 (https://bugzilla.tianocore.org/show_bug.cgi?id=3415)

The GICv3 architecture supports up to 1020 ordinary interrupt
lines. The actual number of interrupts supported is described by the
ITLinesNumber field in the GICD_TYPER register. The total number of
implemented registers is normally calculated as
32*(ITLinesNumber+1). However, maximum value (0x1f) is a special case
since that would indicate that 1024 interrupts are implemented.

Add handling for this special case in ArmGicGetMaxNumInterrupts.

Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>

ArmPkg/Drivers/ArmGic/ArmGicLib.c

@@ -1,6 +1,6 @@
 /** @file
 *
-*  Copyright (c) 2011-2018, ARM Limited. All rights reserved.
+*  Copyright (c) 2011-2021, Arm Limited. All rights reserved.
 *
 *  SPDX-License-Identifier: BSD-2-Clause-Patent
 *
@@ -120,7 +120,14 @@ ArmGicGetMaxNumInterrupts (
   IN  INTN          GicDistributorBase
   )
 {
-  return 32 * ((MmioRead32 (GicDistributorBase + ARM_GIC_ICDICTR) & 0x1F) + 1);
+  UINTN ItLines;
+
+  ItLines = MmioRead32 (GicDistributorBase + ARM_GIC_ICDICTR) & 0x1F;
+
+  //
+  // Interrupt ID 1020-1023 are reserved.
+  //
+  return (ItLines == 0x1f) ? 1020 : 32 * (ItLines + 1);
 }
 
 VOID