rockchip: fix set default serial_{addr,port} in boot script

rockchip: fix set default serial_{addr,port} in boot script

On some SoCs like RK3399, U-Boot no longer export the serial
address in $stdout variable, it has been changed to something like
`stdout=serial,vidconsole` since video supported was added. So
detect $soc variable instead, this also makes the script more clear.

Addtional note: rk3566 is reported as rk3568, rk3582 is reported as
rk3588 in $soc.

Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>
Link: https://github.com/openwrt/openwrt/pull/21610
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>