cmd: kaslrseed: add command to generate value from hwrng

cmd: kaslrseed: add command to generate value from hwrng

Allow the kaslr-seed value in the chosen node to be set from a hardware
rng source.

Tested on a Rockchip PX30 (Odroid Go Advance), you must have loaded
the devicetree first and prepared it for editing. On my device the
workflow goes as follows:

setenv dtb_loadaddr "0x01f00000"
load mmc 0:1 ${dtb_loadaddr} rk3326-odroid-go2.dtb
fdt addr ${dtb_loadaddr}
fdt resize
kaslrseed

and the output can be seen here:
fdt print /chosen
chosen {
        kaslr-seed = <0x6f61df74 0x6f7b996c>;
        stdout-path = "serial2:115200n8";
};

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>