riscv: add RISC-V fields to bdinfo command

The firmware invoking main U-Boot uses

* a0 to pass the boot hart
* a1 to pass a device-tree

Let the bdinfo command print this information, e.g.

    boot hart   = 0x000000000000001b
    firmware fdt= 0x0000000087e00000

The firmware fdt field will only be printed if it is non-zero.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>