powerpc/prom_init: don't use string functions from lib/

When KASAN is active, the string functions in lib/ are doing the
KASAN checks. This is too early for prom_init.

This patch implements dedicated string functions for prom_init,
which will be compiled in with KASAN disabled.

Size of prom_init before the patch:
   text	   data	    bss	    dec	    hex	filename
  12060	    488	   6960	  19508	   4c34	arch/powerpc/kernel/prom_init.o

Size of prom_init after the patch:
   text	   data	    bss	    dec	    hex	filename
  12460	    488	   6960	  19908	   4dc4	arch/powerpc/kernel/prom_init.o

This increases the size of prom_init a bit, but as prom_init is
in __init section, it is freed after boot anyway.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>