[MTD] onenand: Add panic_write function to the onenand driver

Implement the panic_write function for the onenand driver. This waits
for any active command to complete/timeout, performs the write, waits
for it to complete and then returns.

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>