firmware: Add fw_dynamic firmware

This patch provides first-cut implementation of fw_dynamic firmware.

As compared to fw_jump and fw_payload, the fw_dynamic obtains next
address, next mode and OpenSBI options from struct fw_dynamic_info.

The previous booting stage can create struct fw_dynamic_info in memory
and pass address of struct fw_dynamic_info in 'a2' register. Also, the
struct fw_dynamic_info has versioning as well so changes to the struct
fw_dynamic_info can be done in a backward compatible manner.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>