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@wendy
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>