ArmPkg: Prevent SgiId from setting RES0 bits of GICD_SGIR

GICD_SGIR is a 32-bit register, of which INTID is bits [3:0] and Bits
[14:4] is RES0. Since SgiId parameter in the function ArmGicSendSgiTo ()
is UINT8, mask unused bits of SgiId before writing to the GICD_SGIR
register to prevent accidental setting of the RES0 bits.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

ArmPkg/Drivers/ArmGic/ArmGicLib.c

@@ -148,7 +148,9 @@ ArmGicSendSgiTo (
 {
   MmioWrite32 (
     GicDistributorBase + ARM_GIC_ICDSGIR,
-    ((TargetListFilter & 0x3) << 24) | ((CPUTargetList & 0xFF) << 16) | SgiId
+    ((TargetListFilter & 0x3) << 24) |
+    ((CPUTargetList & 0xFF) << 16)   |
+    (SgiId & 0xF)
     );
 }