ARM: dts: imx27-eukrea-mbimxsd27-baseboard: Pass missing unit address

ads7846 is connected to SPI chip select 0, so pass the unit address
accordingly to fix the following build warning with W=1:

arch/arm/boot/dts/imx27-eukrea-mbimxsd27-baseboard.dtb: Warning (unit_address_vs_reg): Node /soc/aipi@10000000/cspi@1000e000/ads7846 has a reg or ranges property, but no unit name

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>