mtd: spi-nor: Add support for sst25wf020a

It is a 256KiB flash with 4 KiB erase sectors
and 64KiB overlay blocks.

This is the one available on Hardkernel's Odroid U3 shield.

Signed-off-by: Alexis Ballier <aballier@gentoo.org>
[Brian: seems like this does NOT require the usual SST_WRITE hacks]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>