ARM: dts: imx28: Fix dcp compatible

ARM: dts: imx28: Fix dcp compatible

Per fsl-dcp.yaml, passing "fsl,imx28-dcp", "fsl,imx23-dcp" is not
valid.

Change it to pass only "fsl,imx28-dcp" to fix the following
schema warning:

crypto@80028000: compatible: 'oneOf' conditional failed, one must be fixed:
	['fsl,imx28-dcp', 'fsl,imx23-dcp'] is too long
	'fsl,imx28-dcp' is not one of ['fsl,imx6sl-dcp', 'fsl,imx6ull-dcp']
	'fsl,imx28-dcp' was expected

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>