FROMLIST: mtd: spi-nor: add support to SST sst26* QSPI memories

This patch adds support to the SST sst26* QSPI memories.
It also adds a new SST_BLOCK_PROTECT flag so spi_nor_scan() sends
a SST Global Block Protection Unlock (98h) command once for all
otherwise later Sector Erase and Page Program commands would fail.

It was tested with sst26vf016b, sst26vf032b, sst26vf032ba and sst26vf064b
on a sama5d2 xplained board at 55.3MHz and 3.3V.

sst26wf040b and sst26wf080b were not tested since they are 1.8V memories
hence don't fit the sama5d2 xplained board requirements.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
(pwclient git-am 9392589)