docs: Document parameters passed to firmware and alignment requirement

This updates documentation to describe parameters passed to firmware
from previous booting stage, and corresponding address alignment
requirement.

This also fixes a typo in fw_dynamic.md (it's => its).

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>