soc: brcmstb: biuctrl: Move to early_initcall

Being called during early_initcall() is early enough that it occurs
before SMP initialization, which is all we care about for the Bus
Interface Unit configuration.

This solves lack of BIU initialization on ARM64 platforms where we do
not have an anchor where to put the BIU initialization (since there are
no machine descriptors).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>