doc: sifive: fu540: Add description for OpenSBI generic platform

OpenSBI generic platform support provides platform specific
functionality based on the FDT passed by previous booting stage.

Depends on OpenSBI commit:
platform: Add generic FDT based platform support
(sha1: f1aa9e54e00006ae70aeac638d5b75093520f65d)

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>