MdeModulePkg EhciPei: Support IoMmu

MdeModulePkg EhciPei: Support IoMmu

V2: Halt HC at EndOfPei.

Update the EhciPei driver to consume IOMMU_PPI to allocate DMA buffer.

If no IOMMU_PPI exists, this driver still calls PEI service to allocate
DMA buffer, with assumption that DRAM==DMA.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>