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

[ Upstream commit 6b23af0783a54efb348f0bd781b7850636023dbb ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>