fastboot: nand: Add pre erase and write hooks

Some devices might need to do some per-partition initialization
(ECC/Randomizer settings change for example) before actually accessing it.

Add some hooks before the write and erase operations to let the boards
define what they need to do if needed.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>