Revert "arm64: dts: Add support for Unisoc's UMS512"

The patch uses the "dt-bindings/clock/sprd,ums512-clk.h header, which
is not merged yet. This caused a build regression, and it means the
patch was not ready to get merged anyway.

This reverts commit 23410de5.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>