ARM: dts: imx6ul: fix qspi node compatible

imx6ul is not compatible to imx6sx, both have different erratas.
Fixes the dt_binding_check warning:
spi@21e0000: compatible: 'oneOf' conditional failed, one must be fixed:
['fsl,imx6ul-qspi', 'fsl,imx6sx-qspi'] is too long
Additional items are not allowed ('fsl,imx6sx-qspi' was unexpected)
'fsl,imx6ul-qspi' is not one of ['fsl,ls1043a-qspi']
'fsl,imx6ul-qspi' is not one of ['fsl,imx8mq-qspi']
'fsl,ls1021a-qspi' was expected
'fsl,imx7d-qspi' was expected

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>