i.MX7D is supported for either the v7-A or the v7-M cores,
but the latter causes a warning:

WARNING: unmet direct dependencies detected for ARM_ERRATA_814220
  Depends on [n]: CPU_V7 [=n]
  Selected by [y]:
  - SOC_IMX7D [=y] && ARCH_MXC [=y] && (ARCH_MULTI_V7 [=n] || ARM_SINGLE_ARMV7M [=y])

Make the select statement conditional.

Fixes: 4562fa4c ("ARM: imx: Enable ARM_ERRATA_814220 for i.MX6UL and i.MX7D")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>