arm: socfpga: sr1500 use environment in common header

This removes the default environment from the sr1500 header
and instead uses the common environment provided in
socfpga_common.h which now uses distro boot.

This board has no upstream devicetree in the kernel source,
so set to socfpga_cyclone5_sr1500.dtb.

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

--
Changes in v2:
 - Remove unneeded CONFIG_BOOTFILE
 - set devicetree name to match socfpga_{fpga model}_{board model}.dts
   pattern