interconnect: icc-rpm: Support child NoC device probe

As shown in downstream DT[1], the System NoC of QCM2290 is modelled
using 4 fab/noc devices: sys_noc + qup_virt + mmnrt_virt + mmrt_virt.
Among those 3 virtual devices, qup is owned by RPM and has no regmap
resource, while mmnrt and mmrt are owned by AP and share the same
regmap as sys_noc.  So it's logical to represent these virtual devices
as child nodes of sys_noc in DT, so that such configuration can be
supported with a couple of changes on qnoc_probe():

- If there are child nodes, populate them.
- If the device descriptor has .regmap_cfg but there is no IOMEM
  resource for the device, use parent's regmap.

[1] https://android.googlesource.com/kernel/msm-extra/devicetree/+/refs/tags/android-11.0.0_r0.56/qcom/scuba-bus.dtsi

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Link: https://lore.kernel.org/r/20211215002324.1727-4-shawn.guo@linaro.org
Signed-off-by: Georgi Djakov <djakov@kernel.org>