ASoC: core: If a platform doesn't have an of_node use parent's node

Support was added to allow location of both CPU and CODEC components
of a DAI link from their parent's of_node if they did not have an
of_node themselves in this commit:

commit 3e0aa8d8 ("ASoC: core: If component doesn't have of_node
                      use parent's node instead")

However this leaves platforms as something of a special case as the
major DAI component that doesn't do this. Since this is useful for MFD
devices which often utilise a single device tree entry for the whole
device, add support for looking up platforms from the parent's of_node
as well.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>