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.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>