riscv: dts: spacemit: Acquire clocks for UART

riscv: dts: spacemit: Acquire clocks for UART

The K1 SoC features two clocks for UART controller, Acquire them
explicitly in the driver. Also it is required to remove the
clock-frequency properties from the uart node, otherwise the new
clock properties are ignored by of_platform_serial_setup() in "8250_of.c".

Reviewed-by: Alex Elder <elder@riscstar.com>
Reviewed-by: Haylen Chu <heylenay@4d2.org>
Link: https://lore.kernel.org/r/20250424-05-dts-clock-v2-2-17d83a705c4c@gentoo.org
Signed-off-by: Yixun Lan <dlan@gentoo.org>