Currently, the xlat region of the on-chip SRAM is always allocated
for all BL images.

The access to the on-chip SRAM is necessary for loading images from
a USB memory device (i.e. when updating firmware), so unneeded for
the usual boot procedure.

To avoid this waste, allocate the xlat region dynamically only for
BL2, and only when it is necessary.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>