powerpc: add support for csum_add()

The C version of csum_add() as defined in include/net/checksum.h gives
the following assembly in ppc32:
       0:       7c 04 1a 14     add     r0,r4,r3
       4:       7c 64 00 10     subfc   r3,r4,r0
       8:       7c 63 19 10     subfe   r3,r3,r3
       c:       7c 63 00 50     subf    r3,r3,r0
and the following in ppc64:
   0xc000000000001af8 <+0>:	add     r3,r3,r4
   0xc000000000001afc <+4>:	cmplw   cr7,r3,r4
   0xc000000000001b00 <+8>:	mfcr    r4
   0xc000000000001b04 <+12>:	rlwinm  r4,r4,29,31,31
   0xc000000000001b08 <+16>:	add     r3,r4,r3
   0xc000000000001b0c <+20>:	clrldi  r3,r3,32
   0xc000000000001b10 <+24>:	blr

include/net/checksum.h also offers the possibility to define an arch
specific function.  This patch provides a specific csum_add() inline
function.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>