x86/iommu: add IOMMU_STRESS Kconfig entry

This Kconfig option is intended to enable various code paths or
parameters in IOMMU implementations to stress test the code and/or the
hardware. This can also be done by disabling optimizations in the code
when this option is switched on.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>