socfpga would load the environment from a file named "barebox.env"
located on the device "/dev/mmc0.1".  Both those names are hard-coded
in the socfpga code and can't be changed.

Barebox supports selecting the location of the environment using a
"barebox,environment" node in device tree's "chosen" node.  And
recently supports specifying that the env should come from a file on
this device.

Change socfpga to use this mechanism by adding the appropriate device
node.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>