poplar: provide more space for kernel image

Adjust mem layout, providing more space for linux kernel image.

This fixes the problem:
ERROR: FDT image overlaps OS image (OS=0x30000000..0x32580000)

Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>