MIPS: hwrng: Add TX4939 RNG driver

This patch adds support for the integrated RNG of the TX4939 SoC.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>