soc/qbman: Fix implicit header dependency now causing build fails

In commit 461a6946 ("iommu: Remove pci.h include from
trace/events/iommu.h") that header shuffle uncovered an implicit
include in this driver, manifesting as:

    CC      drivers/soc/fsl/qbman/qman_portal.o
    drivers/soc/fsl/qbman/qman_portal.c: In function 'qman_portal_probe':
    drivers/soc/fsl/qbman/qman_portal.c:299:2: error: implicit declaration of function 'dma_set_mask'
    drivers/soc/fsl/qbman/qman_portal.c:299:2: error: implicit declaration of function 'DMA_BIT_MASK'
        if (dma_set_mask(dev, DMA_BIT_MASK(40))) {
        ^

on the corenet32_smp_defconfig (and 64 bit respectively.)  The above
commit was singled out via git bisect.

The header it was implictly relying on getting was dma-mapping.h - so
we explicitly add it here.

Fixes: 461a6946 ("iommu: Remove pci.h include from trace/events/iommu.h")
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Scott Wood <oss@buserror.net>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>