powerpc/powernv: Mark function as __noreturn

There is a mismatch between function pnv_platform_error_reboot() definition
and declaration regarding function modifiers. In the declaration part, it
contains the function attribute __noreturn, while function definition
itself lacks it.

This was reported by sparse tool as an error:

  arch/powerpc/platforms/powernv/opal.c:538:6: error: symbol 'pnv_platform_error_reboot' redeclared with different type (originally declared at arch/powerpc/platforms/powernv/powernv.h:11) - different modifiers

I checked and the function is already being considered as being 'noreturn'
by the compiler, thus, I understand this patch does not change any code
being generated.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>