Reapply "iommu: Hide ops.domain_alloc behind CONFIG_FSL_PAMU"

This reverts commit 369d6782.

Pixel has moved away from the ops->domain_alloc() and we can bring back
this upstream commit now.

Bug: 429438232
Cc: Daniel Mentz <danielmentz@google.com>
Cc: Will McVicker <willmcvicker@google.com>
Change-Id: I6bc2f7d6d46c4106a074caaddfc106021e200dd3
Signed-off-by: Carlos Llamas <cmllamas@google.com>