arch: mips: fix unmet dependency for MTD_COMPLEX_MAPPINGS

When CAVIUM_OCTEON_SOC is enabled, and MTD is disabled,
Kbuild gives the following warning:

WARNING: unmet direct dependencies detected for MTD_COMPLEX_MAPPINGS
  Depends on [n]: MTD [=n] && HAS_IOMEM [=y]
  Selected by [y]:
  - CAVIUM_OCTEON_SOC [=y] && <choice>

This is because CAVIUM_OCTEON_SOC selects MTD_COMPLEX_MAPPINGS,
without selecting or depending on MTD, despite MTD_COMPLEX_MAPPINGS
depending on MTD.

Signed-off-by: Julian Braha <julianbraha@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>