dt-bindings: riscv: add Zilsd and Zclsd extension descriptions

dt-bindings: riscv: add Zilsd and Zclsd extension descriptions

Add descriptions for the Zilsd (Load/Store pair instructions) and
Zclsd (Compressed Load/Store pair instructions) ISA extensions
which were ratified in commit f88abf1 ("Integrating load/store
pair for RV32 with the main manual") of the riscv-isa-manual.

Signed-off-by: Pincheng Wang <pincheng.plct@isrc.iscas.ac.cn>
Reviewed-by: Nutty Liu <nutty.liu@hotmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20250826162939.1494021-2-pincheng.plct@isrc.iscas.ac.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>