powerpc: deleted unused symbol CONFIG_SPL_NAND_MINIMAL and enabled some functionality for common SPL

1. The symbol CONFIG_SPL_NAND_MINIMAL is unused, so deleted it.
2. Some functions were unused in the minimal SPL, but it is useful
in the common SPL. So, enabled some functionality for common SPL.

Signed-off-by: Ying Zhang <b40530@freescale.com>
Acked-by: York Sun <yorksun@freescale.com>