ARM: dts: ls1021a-tsn: use generic "jedec,spi-nor" compatible for flash

We cannot list all the possible chips used in different board revisions,
just use the generic "jedec,spi-nor" compatible instead.  This also
fixes dtbs_check error:
['jedec,spi-nor', 's25fl256s1', 's25fl512s'] is too long

Signed-off-by: Li Yang <leoyang.li@nxp.com>
Reviewed-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>