uboot-imx

Define board_quiesce_devices() in a shared location

This undocumented function relies on arch-specific code to declare a nop
weak version. Add the weak function in common code instead to avoid having
to duplicate the same function in each arch.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>