ARM: moxart: fix CPU selection

Moxart uses an FA526 CPU core, which is ARMv4 based, not ARMv4T.
Before moxart, we had no CONFIG_MULTI_V4 option, since no ARMv4 platform
was enabled for multiplatform. This now adds the missing option, which
will give us slightly more efficient code on pure moxart kernels,
because we can build a pure FA526 kernel now rather than a combined
FA526+ARM920T kernel that we used to.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>