powerpc/85xx/p5040: add CONFIG_SYS_PPC64, del CONFIG_SYS_FSL_ELBC_MULTIBIT_ECC

powerpc/85xx/p5040: add CONFIG_SYS_PPC64, del CONFIG_SYS_FSL_ELBC_MULTIBIT_ECC

The P5040 has an e5500 core, so CONFIG_SYS_PPC64 should be defined in
config_mpc85xx.h.  This macro was absent in the initial P5040 patch because
it crossed paths with the patch that introduced the macro.

Also delete CONFIG_SYS_FSL_ELBC_MULTIBIT_ECC, since it's not used in the
upstream U-Boot.  It's a holdover from the SDK.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>