arm: kirkwood: Use Kirkwood common early malloc area for various Kirkwood boards

For Kirkwood boards, it is necessary to have early malloc in DRAM area
when Driver Model for Serial is enabled. This patch removes individual
board settings for early malloc in various Kirkwood boards, and uses the
Kirkwood commonly defined settings in arch/arm/mach-kirkwood/Kconfig.

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