i.MX31: Add hw-random for RNGA

This hw-random driver add support to RNGA hardware found
on some i.MX processors.

Signed-off-by: Alan Carvalho de Assis <acassis@gmail.com>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>