ARM: i.MX31: remove rnga registration as a platform device

On i.MX31 powered boards with OF support Security Random Number
Generator Accelerator RNGA controller is initialized from device tree,
its registration as a platform device is redundant and actually it is
broken due to missing clock information:

  mxc_rnga mxc_rnga: Could not get rng_clk!
  mxc_rnga: probe of mxc_rnga failed with error -2

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>