rockchip: rk356x: Move common uart2 props to rk356x-u-boot.dtsi

rockchip: rk356x: Move common uart2 props to rk356x-u-boot.dtsi

Move uart2 bootph-pre-ram and clock-frequency props from board to SoC
u-boot.dtsi. Regular board device tree already enables the uart2 node,
so status prop is dropped from u-boot.dtsi file.

Also remove unnecessary stdout-path = &uart2, regular board device tree
already provide a stdout-path = "serial2:" value.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>