rockchip64/uboot: increase rng-seed size to make it sufficient for modern linux

[alex3d]

Description

Modern linux requires 32 byte seed to initialise random pool, but u-boot currently provides only 8 bytes. Increase rng-seed size to make linux happy and initialise instantly.
This significantly speeds-up encrypted overlayroot mount (by 5 sec or so), and I expect cryptroot would benefit either.

Without patch:

# dmesg | grep crng
[   12.089286] random: crng init done

With patch:

# dmesg | grep crng
[    0.000000] random: crng init done

How Has This Been Tested?

• Ruild and run on rk3318-box

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code