i2c: brcmstb: Make the driver buildable on BMIPS_GENERIC

The BCM7xxx ARM and MIPS based SoCs share a similar I2C hardware block.

Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>