docs: fix errors/ambiguities in docs for Apollo Lake boards

The documentation for the GPMRB incorrectly made reference to the
up_squared board in its high-speed UART configuration section. We
consolidate the related documentation for all boards based on the
Apollo Lake SoC and adjust the language to be more generic.

Fixes: #18808

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