arm64: dts: visconti: Update the clock providers for UART

Remove fixed clock and source common clock for UART.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Link: https://lore.kernel.org/r/20220510015229.139818-2-nobuhiro1.iwamatsu@toshiba.co.jp/