arm: mvebu: Add DM and OF_CONTROL support to SPL

This patch adds full DM support to the SPL on MVEBU. Currently
only serial is supported. Other drivers will follow.

This patch also adds the necessary config values for the DEBUG UART
to the MVEBU defconfig files. This came in handy while implementing
this DM support.

Additionally, the mvebu specific SPL linker script is removed and
this common one is used instead:

   arch/arm/cpu/u-boot-spl.lds

This common linker script already handles all special cases. No need
to reinvent the wheel for MVEBU here.

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