spl/powerpc: introduce CONFIG_SPL_INIT_MINIMAL

cpu_init_nand.c is renamed to spl_minimal.c as it is not really NAND-specific.

Signed-off-by: Scott Wood <scottwood@freescale.com>
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v2: factor out START, and change cpu_init_nand.c to spl_minimal.c
Cc: Andy Fleming <afleming@freescale.com>