target-ppc: enable PPC_MFTB for 44x

According to PPC440 user manual, PPC 440 supports ``mftb'' even it's a
preserved instruction:

PPC440_UM2013.pdf, p.445, table A-3

when I compile a kernel (2.6.30, bamboo_defconfig/440EP &
canyonlands/460EX), I can see ``mftb'' by using ppc-xxx-objdump
vmlinux

I have also checked the ppc 440x[456], 460S, 464, they also should support mftb.

The following patch enable mftb for all ppc 440 variants, including:
440EP, 440GP, 440x4, 440x5 and 460

Signed-off-by: Baojun Wang <wangbj@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
