arm: kirkwood: Enable uart0 dm-pre-reloc for Pogoplug V4 board

When DM_SERIAL is enabled, the device-tree tag u-boot,dm-pre-reloc is
required for this board to boot over UART with kwboot. Enable this in
kirkwood-pogoplug-series-4-u-boot.dtsi.

Added by Stefan while applying:
Please note that it's not fully understood, why this property really
is needed. Here a link to the discussion about this:

https://lore.kernel.org/r/20230201080210.ypz4nrj4y2igwxz3@pali/



Signed-off-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>