MLK-9920 mtd: qspi: Add ddrsmp parameter to device tree

Since QSPI internal DDR sample point is relevant with board layout,
we can't use same value for all boards. Add ddrsmp parameter to device
tree for i.MX6SX Sabreauto/Sabresd board.

DDRSMP value:
0 ---- i.MX6SX Sabresd board (RevB and RevA)
2 ---- i.MX6SX Sabreauto board

The Sabresd RevA board also needs to reduce clock to 29Mhz according to
the Spansion spec.

Signed-off-by: Ye.Li <B37916@freescale.com>
(cherry picked from commit c9115cc22d836b5b980ca20932a005ea61b20082)