mfd: sprd-sc27xx-spi: Fix symbol 'sprd_pmic_detect_charger_type' was not declared warning

Sparse reports:

 drivers/mfd/sprd-sc27xx-spi.c:59:23: warning: symbol 'sprd_pmic_detect_charger_type' was not declared. Should it be static?

... due to a missing header file.

Cc: Orson Zhai <orsonzhai@gmail.com>
Cc: Chunyan Zhang <zhang.lyra@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>