spl, nand, atmel_nand: add erase one block function

erase one nand block in spl code. keep it simple, as size matters
This is used on the upcoming taurus spl support.

Signed-off-by: Heiko Schocher <hs@denx.de>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: Bo Shen <voice.shen@atmel.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>