powerpc: dts: p2020: Add Linux compatible string and property for eSDHC

Linux kernel eSDHC driver for P2020 requires additional compatible string
fsl,p2020-esdhc and interrupts property. Add them to p2020-post.dtsi file
to make U-Boot board DTS files compatible for Linux kernel.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>