Documentation/features: Add KGDB support on RISC-V

As KGDB has been implemented on RISC-V, update the feature list.

Signed-off-by: Shuli Zhang <zhshuli@aliyun.com>
Link: https://lore.kernel.org/r/20200621123016.9289-1-zhshuli@aliyun.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>