powerpc/powernv: Make some symbols static

Fix sparse warnings:

  arch/powerpc/platforms/powernv/opal-psr.c:20:1:
   warning: symbol 'psr_mutex' was not declared. Should it be static?
  arch/powerpc/platforms/powernv/opal-psr.c:27:3:
   warning: symbol 'psr_attrs' was not declared. Should it be static?
  arch/powerpc/platforms/powernv/opal-powercap.c:20:1:
   warning: symbol 'powercap_mutex' was not declared. Should it be static?
  arch/powerpc/platforms/powernv/opal-sensor-groups.c:20:1:
   warning: symbol 'sg_mutex' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190702131733.44100-1-yuehaibing@huawei.com