powerpc/powernv: Squash sparse warnings in opal-call.c

sparse complains a lot about opal-call.c:

  arch/powerpc/platforms/powernv/opal-call.c:128:1: warning: symbol 'opal_invalid_call' was not declared. Should it be static?
  arch/powerpc/platforms/powernv/opal-call.c:129:1: warning: symbol 'opal_console_write' was not declared. Should it be static?
  arch/powerpc/platforms/powernv/opal-call.c:130:1: warning: symbol 'opal_console_read' was not declared. Should it be static?

Those symbols are forward declared in opal.h, but we can't include that
because the function signatures in opal.h are different. So instead, just
add an extra forward declaration to the OPAL_CALL macro to shut sparse up.

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>