regulator: bcm590xx: Add support for regulators on secondary I2C slave

The bcm590xx MFD driver now exposes a secondary regmap descriptor
making the registers for regulators on the secondary I2C slave address
available.  Add support for GPLDO1-6 and VBUS regulators found within
this register range.

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