staging: hikey9xx: Specify HAS_IOMEM dependency for MFD_HI6421_SPMI

MFD_CORE is selected by MFD_HI6421_SPMI, and MFD_CORE depends on
HAS_IOMEM. If HAS_IOMEM is not set, this can cause a conflict in Kconfig
resolution, yielding the following error:

WARNING: unmet direct dependencies detected for MFD_CORE
  Depends on [n]: HAS_IOMEM [=n]
  Selected by [y]:
  - MFD_HI6421_SPMI [=y] && STAGING [=y] && OF [=y] && SPMI [=y]

By specifying HAS_IOMEM as a dependency for MFD_HI6421_SPMI (as
SPMI_HISI3670 already dows), this issue is resolved, and no such warning
appears when building on architectures without HAS_IOMEM.
Signed-off-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/r/20201119082903.3601758-1-davidgow@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/hikey9xx/Kconfig

@@ -25,6 +25,7 @@ config SPMI_HISI3670
 # to be placed at drivers/mfd
 config MFD_HI6421_SPMI
 	tristate "HiSilicon Hi6421v600 SPMI PMU/Codec IC"
+	depends on HAS_IOMEM
 	depends on OF
 	depends on SPMI
 	select MFD_CORE