arm: kirkwood: lsxl: Add SPI driver model support

arm: kirkwood: lsxl: Add SPI driver model support

This patch shows how to enable driver model support for the LS-CHLv2 and
LS-XHL boards.

There are a couple of open questions:
 - do I need the u-boot,dm-pre-reloc tags in the device tree?
 - should mach/config.h define CONFIG_DM_SEQ_ALIAS?
 - how can we split this patch or are there any other pending patches
   which does the same and I didn't catch these.

This patch is based on the http://git.denx.de/u-boot-marvell.git (master
branch) and needs the following patches, which are still pending:
  https://patchwork.ozlabs.org/patch/909618/
  https://patchwork.ozlabs.org/patch/909617/
  https://patchwork.ozlabs.org/patch/909973/

Signed-off-by: Michael Walle <michael@walle.cc>
Tested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Stefan Roese <sr@denx.de>