mpc85xx: add socrates dts from linux

add socrates device tree from linux:

commit 71ae5fc87c34 ("Merge tag 'linux-kselftest-5.2-rc1' of
git://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest"

)

and added SPDX license identifier.
Did not fix checkpatch warnings:
arch/powerpc/dts/socrates.dts:235: check: Please don't use multiple blank lines
arch/powerpc/dts/socrates.dts:238: error: code indent should use tabs where possible

Also, add me as board maintainer.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>