The "smmu-cb-memtype" is a private property developed by the Hisilicon
driver in the early stage and is not used now. So delete it.

Otherwise, below YAML check warnings are reported:
arch/arm64/boot/dts/hisilicon/hip06-d03.dt.yaml: iommu@a0040000: \
'smmu-cb-memtype' does not match any of the regexes: 'pinctrl-[0-9]+'
arch/arm64/boot/dts/hisilicon/hip07-d05.dt.yaml: iommu@a0040000: \
'smmu-cb-memtype' does not match any of the regexes: 'pinctrl-[0-9]+'
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>