riscv: dts: microchip: mpfs-icicle: Fix serial console

Currently, nothing is output on the serial console, unless
"console=ttyS0,115200n8" or "earlycon" are appended to the kernel
command line.  Enable automatic console selection using
chosen/stdout-path by adding a proper alias, and configure the expected
serial rate.

While at it, add aliases for the other three serial ports, which are
provided on the same micro-USB connector as the first one.

Fixes: 0fa6107e

 ("RISC-V: Initial DTS for Microchip ICICLE board")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>