socfpga: arria10: Wait for fifo empty after writing bitstream

For some reason, on the Mercury+ AA1 module, calling
fpgamgr_wait_early_user_mode immediately after writing the peripheral
bitstream leaves the fpga in a broken state (ddr calibration hangs).
Adding a delay before the first sync word is written seems to fix this.
Inspecting the fpgamgr registers before and after the delay,
imgcfg_FifoEmpty is the only bit that changes. Waiting for this bit
(instead of a hardcoded delay) also fixes the issue.

Signed-off-by: Paweł Anikiel <pan@semihalf.com>
Reviewed-by: Simon Glass <sjg@chromium.org>