mfd: bcm590xx: Update binding with additional BCM59056 regulators

The BCM59056 supports GPLDO1-6 and VBUS regulators in a secondary
I2C slave address space. Add these regulators to the list of valid
regulator node names for BCM59056.

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