soc: brcmstb: biuctrl: exit without warning on non brcmstb platforms

Currently if this driver is included, we get the following warning
on any platforms irrespective of whether it's brcmstb platform or not.
"
 brcmstb: biuctrl: missing BIU control node
 brcmstb: biuctrl: MCP: Unable to disable write pairing!
"

This patch allows to exit early without any warning messages on non
brcmstb platforms as it's meaningless for them.

Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Gregory Fong <gregory.0xf0@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: bcm-kernel-feedback-list@broadcom.com
Fixes: f780429a ("soc: brcmstb: biuctrl: Move to early_initcall")
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
[florian: Add fixes tag, make initcall non fatal]
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>