powerpc: Replace ffz() by equivalent generic function

With the ffz() function as defined in arch/powerpc/include/asm/bitops.h
GCC will not optimise the code in case of constant parameter.

This patch replaces ffz() by the generic function.

The generic ffz(x) expects to never be called with ~x == 0
as written in the comment in include/asm-generic/bitops/ffz.h
The only user of ffz() within arch/powerpc/ is
platforms/512x/mpc5121_ads_cpld.c, which checks if x is not 0xff

For non constant calls, the generated code is doing the same:

unsigned long testffz(unsigned long x)
{
	return ffz(x);
}

On PPC32, before the patch:
00000018 <testffz>:
  18:	7c 63 18 f9 	not.    r3,r3
  1c:	40 82 00 0c 	bne     28 <testffz+0x10>
  20:	38 60 00 20 	li      r3,32
  24:	4e 80 00 20 	blr
  28:	7d 23 00 d0 	neg     r9,r3
  2c:	7d 23 18 38 	and     r3,r9,r3
  30:	7c 63 00 34 	cntlzw  r3,r3
  34:	20 63 00 1f 	subfic  r3,r3,31
  38:	4e 80 00 20 	blr

On PPC32, after the patch:
00000018 <testffz>:
  18:	39 23 00 01 	addi    r9,r3,1
  1c:	7d 23 18 78 	andc    r3,r9,r3
  20:	7c 63 00 34 	cntlzw  r3,r3
  24:	20 63 00 1f 	subfic  r3,r3,31
  28:	4e 80 00 20 	blr

On PPC64, before the patch:
0000000000000030 <.testffz>:
  30:	7c 60 18 f9 	not.    r0,r3
  34:	38 60 00 40 	li      r3,64
  38:	4d 82 00 20 	beqlr
  3c:	7c 60 00 d0 	neg     r3,r0
  40:	7c 63 00 38 	and     r3,r3,r0
  44:	7c 63 00 74 	cntlzd  r3,r3
  48:	20 63 00 3f 	subfic  r3,r3,63
  4c:	7c 63 07 b4 	extsw   r3,r3
  50:	4e 80 00 20 	blr

On PPC64, after the patch:
0000000000000030 <.testffz>:
  30:	38 03 00 01 	addi    r0,r3,1
  34:	7c 03 18 78 	andc    r3,r0,r3
  38:	7c 63 00 74 	cntlzd  r3,r3
  3c:	20 63 00 3f 	subfic  r3,r3,63
  40:	4e 80 00 20 	blr

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