serial_lpuart: make clock failure less verbose

Some device may enable CONFIG_CLK but not still support this clock in
CC, so better use debug() in place of dev_warn() otherwise a lot of
boards will throw useless dev_warn()s.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Reviewed-by: Simon Glass <sjg@chromium.org>