vela: Fix Ethos-U65 maximum address range

 - Changed Ethos-65 AXI port address width from 48 to 40-bits
 - Fixed the use of arena_cache_size in mem_type_size() to cover the
arena as well as the cache memory area

Signed-off-by: Tim Hall <tim.hall@arm.com>
Change-Id: I826462a0cbd0c061cccbc7c83dde446778a2b1ca