Add support for caching, to reduce FLASH wear when writing using small
blocksizes. As we also don't care anymore about heads and tails in case of
partial writes, this greatly simplifies the code for handling writes.

Note: We don't bother caching reads smaller than the FLASH chunk size
(256 KiB).
Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>