mfd: Add Spreadtrum SC27xx series PMICs driver

This patch adds support for Spreadtrum SC27xx series PMIC MFD core, and It
provides communication through the SPI interfaces. The SC27xx series PMICs
contains the following 6 major components:
- DCDCs
- LDOs
- Battery management system
- Audio codec
- User interface function, such as indicator, flash LED
- IC level function, such as power on/off, type-c

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