riscv: dts: canaan: Clean up serial aliases

riscv: dts: canaan: Clean up serial aliases

The SoC-specific k210.dtsi declares aliases for all four serial ports.
However, none of the board-specific DTS files configure pin control for
any but the first serial port, so the last three ports are not usable.

Move the aliases node from the SoC-specific k210.dtsi to the
board-specific DTS files, as these are really board-specific, and retain
the sole port that is usable.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>