sched/doc: supplement CPU capacity with RISC-V

This commit 7d207831 ("dt-bindings: arm: move cpu-capacity to a
shared loation") updates some references about capacity-dmips-mhz
property in this document.

The list of architectures using capacity-dmips-mhz omits RISC-V, so
supplements it here.

Signed-off-by: Song Shuai <suagrfillet@gmail.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com> # English
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230227105941.2749193-1-suagrfillet@gmail.com


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