gpio: pmic-eic-sprd: Move Kconfig to MFD expanders

The Spreadtrum PMIC EIC interrupt controller is part of an
MFD expander and should thus be in the MFD GPIO expander
menu section with the rest. Move it.

Cc: Cixi Geng <cixi.geng1@unisoc.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>