xen/arm: static memory initialization

xen/arm: static memory initialization

This patch introduces static memory initialization, during system boot-up.

The new function init_staticmem_pages is responsible for static memory
initialization.

Helper free_staticmem_pages is the equivalent of free_heap_pages, to free
nr_mfns pages of static memory.

This commit also introduces a new CONFIG_STATIC_MEMORY option to wrap all
static-allocation-related code.

Put asynchronously scrubbing pages of static memory in TODO list.

Signed-off-by: Penny Zheng <penny.zheng@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>