clk: tegra: Use fence_udelay() during PLLU init

This patch uses fence_udelay rather than udelay during PLLU
initialization to ensure writes to clock registers happens before
waiting for specified delay.

Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>