arm: ti: boot: Increase system partition size

Android code base is growing, so since Android "Pie" the size of
system.img grew up to be about 740 MiB. Let's increase system.img to
1 GiB to accommodate for those changes and leave some margin for future
changes. We don't want to make it more than 1 GiB, because we should
keep userdata partition big enough (for user files, like media etc.),
and eMMC size on BeagleBoard-X15 is only 3.5 GiB.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Acked-by: Praneeth Bajjuri <praneeth@ti.com>