rng: rockchip: Add support for rkrng variant

Add support for rkrng variant, used by e.g. RK3528 and RK3576.

Imported from vendor U-Boot linux-6.1-stan-rkr5 tag with minor
adjustments for mainline.

Signed-off-by: Lin Jinhan <troy.lin@rock-chips.com>
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>