Prepare for the BCM63138 ARCH_BCM_63XX migration to ARCH_BCMBCA. Make
I2C_BRCMSTB depending on ARCH_BCMBCA.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Acked-by: Wolfram Sang <wsa@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>