intel-iommu: Mark functions with __init

Mark si_domain_init and iommu_prepare_static_identity_mapping with
__init, to eliminate the following warnings:

WARNING: drivers/pci/built-in.o(.text+0xf1f4): Section mismatch in reference from the function si_domain_init() to the function .init.text:si_domain_work_fn()
The function si_domain_init() references
the function __init si_domain_work_fn().
This is often because si_domain_init lacks a __init
annotation or the annotation of si_domain_work_fn is wrong.

WARNING: drivers/pci/built-in.o(.text+0xe340): Section mismatch in reference from the function iommu_prepare_static_identity_mapping() to the function .init.text:si_domain_init()
The function iommu_prepare_static_identity_mapping() references
the function __init si_domain_init().
This is often because iommu_prepare_static_identity_mapping lacks a __init
annotation or the annotation of si_domain_init is wrong.

Signed-off-by: Matt Kraai <kraai@ftbfs.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>