mkimage: sunxi_egon: add support for riscv

There's now a sun20i family in sunxi, which uses RISC-V CPU.

Add support for making eGON.BT0 image for RISC-V.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>