serial: serial_mtk: add non-DM version for SPL

This patch adds non-DM version for mtk hsuart driver and makes it
compatible with ns16550a driver in configuration.
This is needed in SPL with CONFIG_SPL_DM disabled for reducing size.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
2 files changed