powerpc/8xxx: Enabled hwconfig for memory interleaving

Replace environmental variables memctl_intlv_ctl and ba_intlv_ctl with
hwconfig parameters. The syntax is

    setenv hwconfig "fsl_ddr:ctlr_intlv=<mode>,bank_intlv=<mode>"

The mode values for memory controller interleaving are
    cacheline
    page
    bank
    superbank

The mode values for bank interleaving are
    cs0_cs1
    cs2_cs3
    cs0_cs1_and_cs2_cs3
    cs0_cs1_cs2_cs3

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