plat/arm: Save BL2 descriptors to reserved memory.

plat/arm: Save BL2 descriptors to reserved memory.

On ARM platforms, the BL2 memory can be overlaid by BL31/BL32. The memory
descriptors describing the list of executable images are created in BL2
R/W memory, which could be possibly corrupted later on by BL31/BL32 due
to overlay. This patch creates a reserved location in SRAM for these
descriptors and are copied over by BL2 before handing over to next BL
image.

Also this patch increases the PLAT_ARM_MAX_BL2_SIZE for juno when TBBR
is enabled.

Fixes ARM-Software/tf-issues#626

Change-Id: I755735706fa702024b4032f51ed4895b3687377f
Signed-off-by: Sathees Balya <sathees.balya@arm.com>