distro/tegrademo-mender.conf: update reserved storage settings

Ensure bootloader update compatibility with older revisions
by reserving an additional 950MB of space above the default
provided in meta-mender-tegra.

Without this, upgrading from an older tegrademo-mender build
would result in a boot failure due to bootloader partition
offsets being different from earlier builds.

Signed-off-by: Matt Madison <matt@madison.systems>