arm: mvebu: Espressobin: Make SPI env offset compatible with Marvell's U-Boot

Espressobin board comes with Marvell's U-Boot version where U-Boot env is
stored in SPI at offset 0x3F0000. This patch changes env offset in
Espressobin defconfig file to match Marvell's U-Boot version.

Users who want to use previous or different env offset can still change it
in .config file when compiling U-Boot.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Konstantin Porotchkin <kostap@marvell.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Andre Heider <a.heider@gmail.com>