config: xea: Add limits for SPL and u-boot proper sizes

The XEA board has following hard constraints regarding size of binaries:
- u-boot.sb < 48 KiB
- u-boot.img < 448 KiB

Added values are supposed to avoid exceeding size of binaries during
future u-boot development.

Signed-off-by: Lukasz Majewski <lukma@denx.de>