ramips: lzma-loader: use default uart for rt305x

ramips: lzma-loader: use default uart for rt305x

The rt305x series SOC have two UART devices,
and the one at bus address 0x500 is disabled by default.
Some boards do not even have a pinout for the first one,
so use the same one that the kernel uses at 0xc00 instead.

This allows the lzma-loader printing to be visible
alongside the kernel log in the same console.

Tested-by: Lech Perczak <lech.perczak@gmail.com> # zte,mf283plus
Signed-off-by: Michael Pratt <mcpratt@pm.me>
(cherry picked from commit bc00c78b4338779ca1b7cd08411f76218d1f3205)
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>