mfd: mp2629: Select MFD_CORE to fix build error

MFD_MP2629 should select MFD_CORE to a prevent build error:

ERROR: modpost: "devm_mfd_add_devices" [drivers/mfd/mp2629.ko] undefined!

Fixes: 06081646 ("mfd: mp2629: Add support for mps battery charger")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>