nand_spl_simple: Add a simple NAND read function

To support UBI in SPL we need a simple NAND read function. Add one to
nand_spl_simple and keep it as simple as it goes.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Acked-by: Scott Wood <oss@buserror.net>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>