mfd: Fix unmet dependency warning for MFD_TPS68470

After commit 5d32a665 ("PCI/ACPI: Allow ACPI to be built without
CONFIG_PCI set") dependencies on CONFIG_PCI that previously were
satisfied implicitly through dependencies on CONFIG_ACPI have to be
specified directly.

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

MFD_TPS68470 is an ACPI only device and selects I2C_DESIGNWARE_PLATFORM.

I2C_DESIGNWARE_PLATFORM does not have any configuration today for ACPI
support without CONFIG_PCI set.

For sake of a quick fix this introduces a new mandatory dependency to
the driver which may survive without it. Otherwise we need to revisit
the driver architecture to address this properly.

Fixes: 5d32a665 ("PCI/ACPI: Allow ACPI to be built without CONFIG_PCI set")
Signed-off-by: Sinan Kaya <okaya@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>