ARM: tegra: fix trimslice environment location

Trimslice currently stores its environment at 512KiB into the SPI flash
chip. The U-Boot binary has grown such that the size of the boot image
(which includes the Tegra BCT, padding, and the U-Boot binary) is slightly
larger than 512K now. Consequently, writing the boot image to flash
corrupts the saved environment, and equally, writing to or erasing the
environment will corrupt the bootloader, which in turn will cause the
Tegra boot ROM to enter recovery mode during boot, making it look as if
the system is non-operational. Note that tegra-uboot-flasher writes to
the environment during the flashing process.

Solve this by moving the environment as high as possible in flash. This
will allow the U-Boot binary to roughly double in size before this problem
is hit again, at which point there's nothing we can do anyway since the
binary won't fit into flash.

99% of other Tegra boards store the environment in eMMC and use a negative
value for CONFIG_ENV_OFFSET, which already automatically places the
environment as near the end of boot flash as possible. The 1 remaining
board hard-codes CONFIG_ENV_OFFSET to 2MiB, which allows for plenty more
bloat.

Reported-by: Stephen L Arnold <nerdboy@gentoo.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>