ARM: realview: fix map_desc alignment

REALVIEW_PBX_TILE_GIC_CPU_BASE is not 4KB aligned which causes an
overlapping mapping. Use REALVIEW_PBX_TILE_SCU_BASE instead which is
aligned.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>