arm: mvebu: Enable DM_SERIAL on AXP / A38x boards

This patch enables DM_SERIAL for all ARCH_MVEBU boards (AXP & A38x).

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Luka Perkov <luka.perkov@sartura.hr>