plat/arm: Fix build failure due to increase in BL2 size

plat/arm: Fix build failure due to increase in BL2 size

BL2 size gets increased due to the libfdt library update and
that eventually cause no-optimization build failure for BL2 as below:
aarch64-none-elf-ld.bfd: BL2 image has exceeded its limit.
aarch64-none-elf-ld.bfd: region `RAM' overflowed by 4096 bytes
Makefile:1070: recipe for target 'build/fvp/debug/bl2/bl2.elf' failed
make: *** [build/fvp/debug/bl2/bl2.elf] Error 1

Fixed build failure by increasing BL2 image size limit by 4Kb.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I92a57eb4db601561a98e254b64994bb921a88db3