ArmVirtPkg: build EnrollDefaultKeys.efi

ArmVirtPkg: build EnrollDefaultKeys.efi

Having removed VALID_ARCHITECTURES from
"OvmfPkg/EnrollDefaultKeys/EnrollDefaultKeys.inf", it now makes sense to
reflect the related platform DSC bits from OvmfPkg to ArmVirtPkg.

Build "EnrollDefaultKeys.efi" as part of ArmVirtQemu and ArmVirtQemuKernel
(which are the ArmVirtPkg platforms that include SecureBootConfigDxe too).

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=1747
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Gary Lin <glin@suse.com>