arm: perf: prevent unbind/bind via sysfs

Unbinding and rebinding the ARM PMU driver via sysfs leads to a
warning followed by more errors:
  WARNING: CPU: 0 PID: 217 at kernel/irq/chip.c:1034 irq_modify_status+0x150/0x16c
  ..
  genirq: Flags mismatch irq 19. 00010c04 (arm-pmu) vs. 00010c04 (arm-pmu)
  hw perfevents: unable to request IRQ19 for ARM PMU counters
  hw perfevents: /pmu: failed to register PMU devices!
  armv7-pmu: probe of pmu failed with error -16

The driver is clearly not designed to be removed. Disable bind/
unbind for this driver.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Will Deacon <will.deacon@arm.com>