boards: fvp_base_revc_2xaemv8a: reserve HEAP for GICv3 ITS tables

The ITS needs a number of table, so HEAP must be reserved for runtime
allocation (1MiB is a round and convenient value):
- Redistributor:
* 64K table for all redistributors
* 64k for each redistributors
- ITS:
* 4K collection table aligned on 4K
* 4K x 128 device table aligned on 4K

This makes 11x64K to permit all allocations to success.

Note, will need 64K HEAP_MEM per CPUs added.

This doesn't necessarily include the Interrupt Translation Table,
which are 256bytes aligned tables, for reference a 32 ITEs table
needs 256bytes.
With 14x64K HEAP, up to 116 ITT tables of 32 ITEs can be allocated.

A specific HEAP_MEM_POOL_SIZE as been added to arm64_gicv3_its test
for the fvp_base_revc_2xaemv8a board to satisfy all memory allocation
constraints for 256 ITT tables with 32 ITEs allocations.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>