arm: kirkwood: Convert to DM Serial for various Kirkwood boards

For Kirkwood boards, it is necessary to have early malloc in DRAM area
when Driver Model for Serial is enabled. Please see Michael's patch here:
https://lore.kernel.org/u-boot/20220817193809.1059688-20-michael@walle.cc/



This patch enables the early malloc in DRAM for all Kirkwood boards.
Note that this will work for boards that have either non-DM serial
and DM_SERIAL. Also, add the CONFIG_KIRKWOOD_COMMON option to enable
DM_SERIAL as a common option for boards that have been tested.

Signed-off-by: Tony Dinh <mibodhi@gmail.com>