ARM: dts: imx: change the spi-nor tx

Before commit 0e30f472 ("mtd: spi-nor: add support for DTR protocol"),
for all PP command, it only support 1-1-1 mode, no matter the tx setting
in dts. But after the upper commit, the logic change. It will choose
the best mode(fastest mode) which flash device and spi-nor host controller
both support.

Though the spi-nor device on imx6sx-sdb/imx6ul(l/z)-14x14-evk board
do not support PP-1-4-4/PP-1-1-4, but if tx is 4 in dts file, it will also
impact the read mode selection. For the spi-nor device on the upper mentioned
boards, they support read 1-4-4 mode and read 1-1-4 mode according to the
device internal sfdp register. But qspi host controller do not support
read 1-4-4 mode. so need to set the tx to 1, let the common code finally
select read 1-1-4 mode, PP-1-1-1 mode.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Fixes: 0e30f472 ("mtd: spi-nor: add support for DTR protocol")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>