arm: socfpga: SoCKit use environment in common header

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

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