of: platform: populate /firmware/ node from of_platform_default_populate_init()

Since "/firmware" does not have its own "compatible" property as it's
just collection of nodes representing firmware interface, it's sub-nodes
are not populated during system initialization.

Currently different firmware drivers search the /firmware/ node and
populate the sub-node devices selectively. Instead we can populate
the /firmware/ node during init to avoid more drivers continuing to
populate the devices selectively.

To generalize the solution this patch populates the /firmware/ node
explicitly from of_platform_default_populate_init().

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Tested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>