mfd: Select MFD_CORE if TPS6105X driver is configured

The TPS61050/61052 driver uses MFD core code, yet does not specify the
dependency in Kconfig. If it is the only MFD driver configured, compilation
fails with

ERROR: "mfd_add_devices" [drivers/mfd/tps6105x.ko] undefined!
ERROR: "mfd_remove_devices" [drivers/mfd/tps6105x.ko] undefined!

Fix the problem by adding "select MFD_CORE" to the respective Kconfig entry.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>