arm: K3: Increase default SYSFW image size allocation

The memory allocated to store the FIT image containing SYSFW and board
configuration data is statically defined to the largest size expected.
This was 269000 bytes but now needs to be grown to 276000 to make room
for the signatures attached to the board configuration data on High
Security devices.

Signed-off-by: Andrew F. Davis <afd@ti.com>