binman: Add support for RISC-V OpenSBI fw_dynamic blob

Add an entry for RISC-V OpenSBI's 'fw_dynamic' firmware payload.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Rick Chen <rick@andestech.com>