ARM: dts: nxp: imx6sx: fix esai related warning when do dtb_check

ARM: dts: nxp: imx6sx: fix esai related warning when do dtb_check

Remove undocumented compatible string 'fsl,imx6sx-esai', which never used
in driver sound/soc/fsl/fsl_esai.c.

Remove unused clock-names 'mem'.  Driver never use clock name 'mem'.

Fix below warning:

arch/arm/boot/dts/nxp/imx/imx6sx-sdb.dtb: esai@2024000: compatible:0: 'fsl,imx6sx-esai' is not one of ['fsl,imx35-esai', 'fsl,imx6ull-esai', 'fsl,imx8qm-esai', 'fsl,vf610-esai']
	from schema $id: http://devicetree.org/schemas/sound/fsl,esai.yaml#
arm/arch/arm/boot/dts/nxp/imx/imx6sx-sdb.dtb: esai@2024000: compatible: ['fsl,imx6sx-esai', 'fsl,imx35-esai'] is too long
	from schema $id: http://devicetree.org/schemas/sound/fsl,esai.yaml#
arm/arch/arm/boot/dts/nxp/imx/imx6sx-sdb.dtb: esai@2024000: clocks: [[2, 239], [2, 240], [2, 152], [2, 239], [2, 196]] is too long
	from schema $id: http://devicetree.org/schemas/sound/fsl,esai.yaml#
arm/boot/dts/nxp/imx/imx6sx-sdb.dtb: esai@2024000: clock-names:1: 'extal' was expected
	from schema $id: http://devicetree.org/schemas/sound/fsl,esai.yaml#
arch/arm/boot/dts/nxp/imx/imx6sx-sdb.dtb: esai@2024000: clock-names:2: 'fsys' was expected
	from schema $id: http://devicetree.org/schemas/sound/fsl,esai.yaml#
arch/arm/boot/dts/nxp/imx/imx6sx-sdb.dtb: esai@2024000: clock-names:3: 'spba' was expected
	from schema $id: http://devicetree.org/schemas/sound/fsl,esai.yaml#
arch/arm/boot/dts/nxp/imx/imx6sx-sdb.dtb: esai@2024000: clock-names: ['core', 'mem', 'extal', 'fsys', 'spba'] is too long

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>