platform: generic: Allow platform_override to perform firmware init

We add a generic platform override callback to allow platform specific firmware init.

Signed-off-by: Wei Liang Lim <weiliang.lim@starfivetech.com>
Reviewed-by: Chee Hong Ang <cheehong.ang@starfivetech.com>
Reviewed-by: Jun Liang Tan <junliang.tan@starfivetech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>