arm: socfpga: C5 SoCDK use environment in common header

This removes the default environment from the C5 SoCDK headers
and instead uses the common environment provided in
socfpga_common.h which now uses distro boot.

In addition to the above, 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 v2:
 - Remove unneeded CONFIG_BOOTFILE