PCI: tegra: limit to MMU build only

This driver rely on MMU for virtual address space optimisations (see
comment around tegra_pcie_bus_alloc()) and breaks NOMMU build, thus
add dependency on MMU.

Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>