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

Documentation/features/debug/kgdb/arch-support.txt

@@ -23,7 +23,7 @@
     |    openrisc: | TODO |
     |      parisc: |  ok  |
     |     powerpc: |  ok  |
-    |       riscv: | TODO |
+    |       riscv: |  ok  |
     |        s390: | TODO |
     |          sh: |  ok  |
     |       sparc: |  ok  |