powerpc 8xx: defconfig: slice by 4 is more efficient than the default slice by 8 on Powerpc 8xx.

On PPC_8xx, CRC32_SLICEBY4 is more efficient (almost twice) than CRC32_SLICEBY8,
as shown below:

With CRC32_SLICEBY8:
[    1.109204] crc32: CRC_LE_BITS = 64, CRC_BE BITS = 64
[    1.114401] crc32: self tests passed, processed 225944 bytes in 15118910 nsec
[    1.130655] crc32c: CRC_LE_BITS = 64
[    1.134235] crc32c: self tests passed, processed 225944 bytes in 4479879 nsec

With CRC32_SLICEBY4:
[    1.097129] crc32: CRC_LE_BITS = 32, CRC_BE BITS = 32
[    1.101878] crc32: self tests passed, processed 225944 bytes in 8616242 nsec
[    1.116298] crc32c: CRC_LE_BITS = 32
[    1.119607] crc32c: self tests passed, processed 225944 bytes in 3289576 nsec

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