powerpc/mpc85xx: Make NMG_CPU_A011 workaround conditional

This erratum applies to the following SoCs:
P4080 rev 1.0, 2.0, fixed in rev 3.0
P2041 rev 1.0, 1.1, fixed in rev 2.0
P3041 rev 1.0, 1.1, fixed in rev 2.0.

Workaround for erratum NMG_CPU_A011 is enabled by default. This workaround
may degrade performance. P4080 erratum CPU22 shares the same workaround.
So it is always enabled for P4080. For other SoCs, it can be disabled by
hwconfig with syntax:

fsl_cpu_a011:disable

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>