clk: tegra: dfll: round down voltages based on alignment

When generating the OPP table, the voltages are round down with the
alignment from the regulator. The alignment should be applied for
voltages look up as well.

Based on the work of Penny Chiu <pchiu@nvidia.com>.

Signed-off-by: Joseph Lo <josephl@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>