mfd: Kconfig: Add missing Kconfig dependency for TPS65086

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>

drivers/mfd/Kconfig

@@ -1294,6 +1294,7 @@ config TPS6507X
 
 config MFD_TPS65086
 	tristate "TI TPS65086 Power Management Integrated Chips (PMICs)"
+	select MFD_CORE
 	select REGMAP
 	select REGMAP_IRQ
 	select REGMAP_I2C