soc: bcm: brcmstb: Be multi-platform compatible

We were making a bunch of wrong assumptions that turned out to blow out
on non-Broadcom STB platforms:

- we would return -ENODEV from brcmstb_soc_device_early_init() if we
  could not find the sun_top_ctrl device node, this is not an error
  in the context of a multi-platform kernel
- we would still try to register the Broadcom STB SoC device, even if we
  are not running on such a platform

While at it, also fix the sun_top_ctrl device_node leaks while we change
the flow of brcmstb_soc_device_init() and
brcmstb_soc_device_early_init().

Fixes: f780429a ("soc: brcmstb: biuctrl: Move to early_initcall")
Signed-off-by: Thierry Reding <treding@nvidia.com>
[florian: Combine all of Thierry's patch in one go for easier review]
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>