MinPlatformPkg/Acpi: MADT NMI default flag set to Edge-Triggered

1. Default APIC NMI structure in MADT changed to expose
   Level-Triggered interrupts.
2. x2APIC NMI structure won't be exposed to OS if x2APIC is not enabled.

Cc: Michael Kubacki <michael.a.kubacki@intel.com>
Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Liming Gao <liming.gao@intel.com>

Signed-off-by: Ankit Sinha <ankit.sinha@intel.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Michael Kubacki <michael.a.kubacki@intel.com>