soc: brcmstb: fix warning from missing include

The brcmstb_biuctrl_init() is defined in the soc specific header
file, but wasn't included in the driver file. Fix the following
warning by including <linux/soc/brcmstb/brcmstb.h> in the driver:

drivers/soc/brcmstb/biuctrl.c:101:13: warning: symbol 'brcmstb_biuctrl_init' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>