dts: spacemit: set console baud rate on bpif3

dts: spacemit: set console baud rate on bpif3

Because the default console's baud rate is not set, defconfig kernels do
not have any serial output on this platform. Set the baud rate to
115200, matching what is used by U-Boot etc on this platform.

Suggested-by: Vivian Wang <wangruikang@iscas.ac.cn>
Fixes: d60d57ab6b2a8 ("riscv: dts: spacemit: add Banana Pi BPI-F3 board device tree")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Yixun Lan <dlan@kernel.org>
Link: https://lore.kernel.org/r/20260430-reword-overstep-3be08b7eab25@spud
Signed-off-by: Yixun Lan <dlan@kernel.org>