lib: sbi_platform: expose hart_features to extension_init callback

The platform-specific extension_init callback is supposed to
set specific things for the platform opensbi is running on.

So it's also the right place to override specific hart_features
if needed - when it's know that autodetection has provided
wrong results for example.

Suggested-by: Atish Patra <atishp@atishpatra.org>
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>