[POWERPC] Change mpc52xx Kconfig entries to match other freescale parts

[Sylvain Munaut]

Grant Likely wrote:
> All other Freescale parts use Kconfig defines of the form: PPC_8?xx
> The MPC52xx is currently in the form: PPC_MPC52xx.  This commit changes
> it to PPC_52xx for consistency sake.
>
> Signed-off-by: Grant Likely <grant.likely at secretlab.ca>
>
>   
Acked-by: Sylvain Munaut <tnt at 246tNt.com>