mfd: sc27xx: Add USB charger type detection support

The Spreadtrum SC27XX series PMICs supply the USB charger type detection
function, and related registers are located on the PMIC global registers
region, thus we implement and export this function in the MFD driver for
users to get the USB charger type.

Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>