ARM: dts: imx6ul: fix keypad compatible

According to binding, the compatible shall only contain imx6ul and imx21
compatibles. Fixes the dt_binding_check warning:
keypad@20b8000: compatible: 'oneOf' conditional failed, one must be fixed:
['fsl,imx6ul-kpp', 'fsl,imx6q-kpp', 'fsl,imx21-kpp'] is too long
Additional items are not allowed ('fsl,imx6q-kpp', 'fsl,imx21-kpp' were
unexpected)
Additional items are not allowed ('fsl,imx21-kpp' was unexpected)
'fsl,imx21-kpp' was expected

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