platform: generic: add extensions_init handler and platform-override

Init of non-standard extensions is a platform-specific thing,
so allow generic platforms to do this via a platform-override.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>