mfd: bcm590xx: Add support for secondary I2C slave address

BCM590xx utilizes a secondary I2C slave address to access additional
register space. Add support for the secondary address space by
instantiating a dummy I2C device with the appropriate secondary
I2C slave address. Also expose a secondary regmap register space so
that MFD drivers can access this secondary i2c slave address space.

Signed-off-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>