mfd: Don't ask about the TPS65912 core driver in Kconfig

The user has to select the I2C and SPI drivers individually and they select
the core driver for the device so there's no point in presenting the user
with an option for the core driver.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>