powerpc: boot: add strrchr function

libfdt gained a new dependency on strrchr, so copy the implementation
from lib/string.c. Most of the string functions are in assembly, but
stdio.c already has strnlen, so add strrchr there.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Rob Herring <robh@kernel.org>