ARM: imx: deconstruct mxc_rnga initialization

The rnga platform device is initialized for all imx31 machines
from its own initcall, but is never initialized anywhere else.

This moves the platform device creation into both the imx31
dt and non-dt machine init sequences, which has basically the
exact same effect as before, but makes it more obvious what
is going on, while reducing the amount of code and removing
the last user of cpu_is_mx31().

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>