dt-bindings: riscv: explicitly mention assumption of Zicsr & Zifencei support

The dt-binding was defined before the extraction of csr access and
fence.i into their own extensions, and thus the presence of the I
base extension implies Zicsr and Zifencei.
There's no harm in adding them obviously, but for backwards
compatibility with DTs that existed prior to that extraction, software
is unable to differentiate between "i" and "i_zicsr_zifencei" without
any further information.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230427-fence-blurred-c92fb69d4137@wendySigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -86,6 +86,12 @@ properties:
       User-Level ISA document, available from
       https://riscv.org/specifications/
 
+      Due to revisions of the ISA specification, some deviations
+      have arisen over time.
+      Notably, riscv,isa was defined prior to the creation of the
+      Zicsr and Zifencei extensions and thus "i" implies
+      "zicsr_zifencei".
+
       While the isa strings in ISA specification are case
       insensitive, letters in the riscv,isa string must be all
       lowercase to simplify parsing.