Enable mapping higher physical address

Current ATF uses a direct physical-to-virtual mapping, that is, a physical
address is mapped to the same address in the virtual space. For example,
physical address 0x8000_0000 is mapped to 0x8000_0000 virtual. This
approach works fine for FVP as all its physical addresses fall into 0 to
4GB range. But for other platform where all I/O addresses are 48-bit long,
If we follow the same direct mapping, we would need virtual address range
from 0 to 0x8fff_ffff_ffff, which is about 144TB. This requires a
significant amount of memory for MMU tables and it is not necessary to use
that much virtual space in ATF.

The patch is to enable mapping a physical address range to an arbitrary
virtual address range (instead of flat mapping)
Changed "base" to "base_va" and added "base_pa" in mmap_region_t and
modified functions such as mmap_add_region and init_xlation_table etc.
Fixes ARM-software/tf-issues#158