configs: th1520_lpi4a: Add UART clock frequency

The BROM of TH1520 always initializes UART0's parent clock and
configures the baudrate to 115200. Describe the clock frequency to make
UART function correctly in SPL without introducing CCF.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
1 file changed