virtio-iommu: Remove invalid requirement about padding

This reference to 'padding' is a leftover from a previous draft of the
virtio-iommu device. The field doesn't exist anymore, remove the
requirement.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

virtio-iommu.tex

@@ -106,8 +106,6 @@ \subsection{Device configuration layout}\label{sec:Device Types / IOMMU Device /
 
 \devicenormative{\subsubsection}{Device configuration layout}{Device Types / IOMMU Device / Device configuration layout}
 
-The device SHOULD set \field{padding} to zero.
-
 The device MUST set at least one bit in \field{page_size_mask}, describing
 the page granularity. The device MAY set more than one bit in
 \field{page_size_mask}.