pbl: use "wait" command instead of "flush" command

PBL flush command is restricted to CCSR memory space. So use WAIT
PBI command to provide enough time for data to get flush in
target memory.

Signed-off-by: Zhao Qiang <qiang.zhao@nxp.com>
[York Sun: rewrap commit message]
Reviewed-by: York Sun <york.sun@nxp.com>