boards/up_squared: soc/apollo_lake: enable UARTs per board, not SoC

Which UARTs are broken out from the SoC on a particular board is
board-specific; don't enable UARTs blindly in the SoC Kconfig.

Also, the default UART options are specified in the driver Kconfig, so
the same defaults specified in the SoC Kconfig are redundant. Removed.

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