iommu/io-pgtable: fix all kernel-doc warnings in io-pgtable.h

iommu/io-pgtable: fix all kernel-doc warnings in io-pgtable.h

Avoid kernel-doc warnings in io-pgtable.h:
- use the correct struct member names or kernel-doc format
- add a missing struct member description
- add a missing function return comment section

Warning: include/linux/io-pgtable.h:187 struct member 'coherent_walk' not
 described in 'io_pgtable_cfg'
Warning: include/linux/io-pgtable.h:187 struct member 'arm_lpae_s1_cfg' not
 described in 'io_pgtable_cfg'
Warning: include/linux/io-pgtable.h:187 struct member 'arm_lpae_s2_cfg' not
 described in 'io_pgtable_cfg'
Warning: include/linux/io-pgtable.h:187 struct member 'arm_v7s_cfg' not
 described in 'io_pgtable_cfg'
Warning: include/linux/io-pgtable.h:187 struct member 'arm_mali_lpae_cfg'
 not described in 'io_pgtable_cfg'
Warning: include/linux/io-pgtable.h:187 struct member 'apple_dart_cfg' not
 described in 'io_pgtable_cfg'
Warning: include/linux/io-pgtable.h:187 struct member 'amd' not described
 in 'io_pgtable_cfg'
Warning: include/linux/io-pgtable.h:223 struct member
 'read_and_clear_dirty' not described in 'io_pgtable_ops'
Warning: include/linux/io-pgtable.h:237 No description found for return
 value of 'alloc_io_pgtable_ops'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>