x86: minnowmax: Adjust U-Boot environment address in SPI flash

x86: minnowmax: Adjust U-Boot environment address in SPI flash

Currently U-Boot environment address is at offset 0x7fe00 of a 8MB
SPI flash. When creating a partial u-boot.rom image without flash
descriptor and ME firmware, U-Boot actually occupies the last 1MB
of the flash, and reprograming U-Boot causes previous environment
settings get lost which is not convenient during testing.

Adjust the environment address to 0x6ef000 instead (before the MRC
cache data region in the flash).

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>