dt-bindings: perf: riscv,pmu: fix property dependencies

Seemingly I mis-implemented the dependencies here. The OpenSBI docs only
point out that the "riscv,event-to-mhpmcounters property is mandatory if
riscv,event-to-mhpmevent is present". It never claims that
riscv,event-to-mhpmcounters requires riscv,event-to-mhpmevent.

Drop the dependency of riscv,event-to-mhpmcounters on
riscv,event-to-mhpmevent.

Fixes: 7e38085d ("dt-bindings: riscv: add SBI PMU event mappings")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20230404-tractor-confusing-8852e552539a@spud


Signed-off-by: Rob Herring <robh@kernel.org>