riscv: dts: sophgo: add uart clock for Sophgo CV1800 series SoC

riscv: dts: sophgo: add uart clock for Sophgo CV1800 series SoC

Add missing clocks of uart node for CV1800B and CV1812H.

Reviewed-by: Chen Wang <unicorn_wang@outlook.com>
Link: https://lore.kernel.org/r/IA1PR20MB4953198222C3ABC2A2B6DE21BB262@IA1PR20MB4953.namprd20.prod.outlook.com
Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>