x86, AMD IOMMU: replace self made size parsing with memparse call

This patch replaces the self-made parsing of the amd_iommu_size option with the
generic memparse function call.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: iommu@lists.linux-foundation.org
Cc: bhavna.sarathy@amd.com
Cc: robert.richter@amd.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>