xen/arm: vgic: Use dynamic memory allocation for vgic_rdist_region

xen/arm: vgic: Use dynamic memory allocation for vgic_rdist_region

The number of Redistributor regions allowed for dom0 is hardcoded
to a define MAX_RDIST_COUNT which is 4. Some systems, especially
latest server chips, may have more than 4 redistributors. Either we
have to increase MAX_RDIST_COUNT to a bigger number or allocate
memory based on the number of redistributors that are found in MADT
table. In the worst case scenario, the macro MAX_RDIST_COUNT should
be equal to CONFIG_NR_CPUS in order to support per CPU Redistributors.

Increasing MAX_RDIST_COUNT has a effect, it blows 'struct domain'
size and hits BUILD_BUG_ON() in domain build code path.

struct domain *alloc_domain_struct(void)
{
    struct domain *d;
    BUILD_BUG_ON(sizeof(*d) > PAGE_SIZE);
    d = alloc_xenheap_pages(0, 0);
    if ( d == NULL )
        return NULL;
...

This patch uses the second approach to fix the BUILD_BUG().

Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>