configs: p3450: reduce size of Jetson Nano u-boot.bin

configs: p3450: reduce size of Jetson Nano u-boot.bin

The Jetson Nano contains all it's firmware on a 4Mb SPI flash, the
allocated size in that flash for U-Boot is 753664 bytes so we need to
ensure the u-boot.bin doesn't exceed that else it will fail.

Add a BOARD_SIZE_LIMIT and drop a few large, and somewhat esoteric, options
to bring us back under that limit.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Svyatoslav Ryhel <clamor95@gmail.com>
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>