mtd: spi-nor: allow dual/quad reads on S25FL129P

According to the datasheet[1], both S25FL129P0 (256kB sectors) and
S25FL129P1 (64kB sectors) support dual read, quad read, dual i/o read
and quad i/o read.

I have verified dual read to be working for S25FL129P1 on a dual
capable spi controller. Quad as well as S25FL129P0 is untested, lacking
hardware to verify.

[1] http://www.spansion.com/Support/Datasheets/S25FL129P_00.pdf



Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>