tegra: Add a delay in clock_start_periph_pll()

This function enables a peripheral clock and then immediately sets its
divider. Add a delay to allow the clock to settle first. This matches the
delay in other places which do a similar thing.

Without this, the I2S device on Nyan does not init properly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>