mfd: sprd: Add one syscon cell to access PMIC global registers

There are some global registers in Spreadtrum sc27xx PMICs, which will
be accessed by other drivers. So this patch adds one syscon cell to
help to access the PMIC's global registers.

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