hwrng: omap - update Kconfig help description

omap-rng also supports Marvell Armada 7k/8k SoCs, but no mention of this
is made in the help text, despite the dependency being added. Explicitly
mention these SoCs in the help description so people know that it covers
more than just TI SoCs.

Fixes: 38321242

 ("hwrng: omap - Add device variant for SafeXcel IP-76 found in Armada 8K")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>