soc/apollo_lake: boards/up_squared: move UART fixups to SoC

The UART references in dts_fixup.h are actually SoC-specific, not
board-specific, so they are moved. Since this leaves the board fixups
empty, the file is removed.

The SoC fixups are expanded to include the additional two ports that
are present on some revisions of the Apollo Lake.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>