drivers: serial: lpuart: Enable SG mode of LPUART

New implemented eDMA loop SG mode can be used to lpuart driver to fix
the conflict of "Done" bit issue which cause uart_async_api test failed
when SG mode is enabled.

Also, this loop SG mode works well even with a high bandrate(test done
on 2000000bps).
Fixes: #78291

Signed-off-by: Raymond Lei <raymond.lei@nxp.com>