mfd: sprd: Populate sub-devices defined in DT

SC27XX-SPI added subdevices according to a pre-defined mfd_cell array,
no matter these devices were really included on board. In this patch,
switch to use devm_of_platform_populate() for adding sub-devices which
are defined in devicetree.

Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>