arm: socfpga: A5 SoCDK use environment in common header

This removes the default environment from the A5 socdk headers
and instead uses the common environment provided in
socfpga_common.h which now uses distro boot.

Add support to boot from the custom a2 type partition.

Change default devicetree name to match devicetree name in
upstream kernel source.

Signed-off-by: Dalon Westergreen <dwesterg@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>

--
Changes in v3:
 - Fix small typo in defconfig, missing "C"
Changes in v2:
 - Remove unneeded CONFIG_BOOTFILE
 - Fix dtb name

a5config test

Signed-off-by: Dalon Westergreen <dwesterg@gmail.com>