memory: tegra: Make IRQ support opitonal

Make IRQ support optional to help unify the Tegra186 memory controller
driver with this one.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20210602163302.120041-9-thierry.reding@gmail.com
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>