arm64: dts: Add SC2731 PMIC dts file for Spreadtrum SC9860

The Spreadtrum SC9860 platform has one SC2731 PMIC, and the SC2731
PMIC integrates all mobile handset power management, audio codec,
battery management and user interface support function in a single
chip.

This patch adds the SC2731 dts file, as well as adding the RTC and
regulator device node for this PMIC.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>