dt-bindings: riscv: explicitly mention assumption of Zicntr & Zihpm support

Similar to commit 41ebfc91

 ("dt-bindings: riscv: explicitly mention
assumption of Zicsr & Zifencei support"), the Zicntr and Zihpm
extensions also used to be part of the base ISA but were removed after
the bindings were merged. Document the assumption of their presence in
the base ISA.

Suggested-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230607-rerun-retinal-5e8ba89e98f1@spud
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>