soc: brcmstb: Add Bus Interface Unit control setup

Broadcom STB SoCs (brcmstb) require an early setup of their Bus
Interface Unit control register, this needs to happen before SMP is
brought up because it affects how the CPU complex will be interfaced to
the memory controller.

Add support code which properly initializes the BIU registers based on
whether "brcm,write-pairing" is present in Device Tree, and take care of
saving and restoring credit register settings during system-wide
suspend/resume operations.

Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>