[MTD] Add mtd panic_write function pointer

MTDs are well suited for logging critical data and the mtdoops driver
allows kernel panics/oops to be written to flash in a blackbox flight
recorder fashion allowing better debugging and analysis of crashes.

Any kernel oops in user context can be easily handled since the kernel
continues as normal and any queued mtd writes are scheduled. Any kernel
oops in interrupt context results in a panic and the delayed writes will
not be scheduled however. The existing mtd->write function cannot be
called in interrupt context so these messages can never be written to
flash.

This patch adds a panic_write function pointer that drivers can
optionally implement which can be called in interrupt context. It is
only intended to be called when its known the kernel is about to panic
and we need to write to succeed. Since the kernel is not going to be
running for much longer, this function can break locks and delay to
ensure the write succeeds (but not sleep).

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