serial: add build-time sanity check of CONFIG_SERIAL_RX_BUFFER_SIZE

serial: add build-time sanity check of CONFIG_SERIAL_RX_BUFFER_SIZE

The help text says it must be a power of 2, and the implementation
does rely on that. Enforce it.

A violation gives a wall of text, but the last few lines should be
reasonably obvious:

drivers/serial/serial-uclass.c:334:9: note: in expansion of macro ‘BUILD_BUG_ON_NOT_POWER_OF_2’
  334 |         BUILD_BUG_ON_NOT_POWER_OF_2(CONFIG_SERIAL_RX_BUFFER_SIZE);

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>