INTEL_SOC_PMIC, INTEL_SOC_PMIC_CHTWC and MFD_TPS68470 select the
I2C_DESIGNWARE_PLATFORM without its dependencies making it possible to see
warning and build error like below:

WARNING: unmet direct dependencies detected for I2C_DESIGNWARE_PLATFORM
  Depends on [n]: I2C [=y] && HAS_IOMEM [=y] && (ACPI [=y] && COMMON_CLK [=n] || !ACPI [=y])
  Selected by [y]:
  - MFD_TPS68470 [=y] && HAS_IOMEM [=y] && ACPI [=y] && I2C [=y]=y

/usr/bin/ld: drivers/i2c/busses/i2c-designware-platdrv.o: in function `dw_i2c_plat_resume':
i2c-designware-platdrv.c:(.text+0x62): undefined reference to `i2c_dw_prepare_clk'
/usr/bin/ld: drivers/i2c/busses/i2c-designware-platdrv.o: in function `dw_i2c_plat_suspend':
i2c-designware-platdrv.c:(.text+0x9a): undefined reference to `i2c_dw_prepare_clk'
/usr/bin/ld: drivers/i2c/busses/i2c-designware-platdrv.o: in function `dw_i2c_plat_probe':
i2c-designware-platdrv.c:(.text+0x41c): undefined reference to `i2c_dw_prepare_clk'
/usr/bin/ld: i2c-designware-platdrv.c:(.text+0x438): undefined reference to `i2c_dw_read_comp_param'
/usr/bin/ld: i2c-designware-platdrv.c:(.text+0x545): undefined reference to `i2c_dw_probe'
/usr/bin/ld: i2c-designware-platdrv.c:(.text+0x727): undefined reference to `i2c_dw_probe_slave'

Fix this by making above options to depend on I2C_DESIGNWARE_PLATFORM
being built-in. I2C_DESIGNWARE_PLATFORM is a visible symbol with
dependencies so in general the select should be avoided.

Fixes: acebcff9 ("mfd: intel_soc_pmic: Select designware i2c-bus driver")
Fixes: de85d79f ("mfd: Add Cherry Trail Whiskey Cove PMIC driver")
Fixes: 9bbf6a15 ("mfd: Add support for TPS68470 device")
Cc: Stable <stable@vger.kernel.org> # v4.14+
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Lee Jones <lee.jones@linaro.org>