soc: bcm: brcmstb: biuctrl: Register writes require a barrier

The BIUCTRL register writes require that a data barrier be inserted
after comitting the write to the register for the block to latch in the
recently written values. Reads have no such requirement and are not
changed.

Fixes: 34642650 ("soc: Move brcmstb to bcm/brcmstb")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>