iommu: arm-smmu: Remove the qcom,skip-init option

Previous changes have added support for detecting which SMR registers and
context banks are initialized by the bootloader, and may already be in use
by hardware. Thus, the qcom,skip-init option is now redundant.

Change-Id: Ic89628d8d09187f007ab21ab2ee74c7f68b4ae42
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>