powerpc/epapr: Move register keyword at the beginning of declaration

Fix warning for all register unsigned long (0,3-12) that appear during W=1
compilation:

./arch/powerpc/include/asm/epapr_hcalls.h:479:2: warning: ‘register’ is not at beginning of declaration [-Wold-style-declaration]
  unsigned long register r[\d] asm("r[\d]");

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>