dm: core: Scan "/firmware" node by default

All Linux firmware drivers are put under "/firmware" node
and it has support to populate "/firmware" node by default.

u-boot and Linux can share same DTB. In this case, driver
probe for devices under "/firmware" will not be invoked
as "/firmware" does not have its own "compatible" property.

This patch scans "/firmware" node by default like "/clocks".

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>