firmware: turris-mox-rwtm: show message about HWRNG registration

Currently it is hard to determinate if on Armada 3720 device is HWRNG
by running kernel accessible or not. So print information message into
dmesg when HWRNG is available and registration was successful.

Fixes: 389711b3 ("firmware: Add Turris Mox rWTM firmware driver")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>