Increase maximum size of BL2 image

Increase maximum size of BL2 image

Increased the maximum size of BL2 image in order to
accommodate the BL2 image when TF-A build with no compiler
optimization for ARM platform.

Note: As of now, "no compiler optimization" build works
only when TRUSTED_BOOT_BOARD option is set to 0.

This change is verified using below CI configuration:
1. juno-no-optimize-default:juno-linux.uboot
2. fvp-no-optimize-default,fvp-default:fvp-tftf-fip.tftf-aemv8a-debug

Change-Id: I5932621237f8acd1b510682388f3ba78eae90ea4
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>