lib/alloc_page: Add shared page allocation support

Add support for allocating "pages" that can be shared with the host.
Or in other words, decrypted pages. This is achieved by adding hooks for
setting a memory region as "encrypted" or "decrypted", which can be overridden
by the architecture specific backends.

Also add a new flag - FLAG_SHARED - for allocating shared pages.

The page allocation/free routines get a "_shared_" variant too.
These will be later used for Realm support and tests.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>