MTF_CORE should be enabled when driver is enabled.

Without this patch you can configure:

CONFIG_MFD_CORE is not set
  CONFIG_MFD_TPS65086=y

... which ends up with compilation error:
drivers/mfd/tps65086.o: In function `tps65086_probe':
drivers/mfd/tps65086.c:110: undefined reference to `mfd_add_devices'
drivers/mfd/tps65086.c:110:(.text+0x128): relocation truncated to fit: \
  R_AARCH64_CALL26 against undefined symbol `mfd_add_devices'
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>