Added the definitions `PLAT_PHY_ADDR_SPACE_SIZE` and
`PLAT_VIRT_ADDR_SPACE_SIZE` which specify respectively the physical
and virtual address space size a platform can use.

`ADDR_SPACE_SIZE` is now deprecated. To maintain compatibility, if any
of the previous defines aren't present, the value of `ADDR_SPACE_SIZE`
will be used instead.

For AArch64, register ID_AA64MMFR0_EL1 is checked to calculate the
max PA supported by the hardware and to verify that the previously
mentioned definition is valid. For AArch32, a 40 bit physical
address space is considered.

Added asserts to check for overflows.

Porting guide updated.

Change-Id: Ie8ce1da5967993f0c94dbd4eb9841fc03d5ef8d6
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>