fix(lib/realm): convert assertions to proper checks

The functions `addr_is_contained()` and `region_is_contained()`
had assertions to validate the sanity of input arguments. But it was
noticed that in some cases, it was possible for this sanity to be
violated at runtime, thus triggering the assertion.

One such instance is in `smc_realm_create()` where it was noticed
that the arguments `rtt_base` and `rtt_base + rtt_num_start * GRANULE_SIZE`
can overflow and violate the sanity of the arguments passed to
`addr_is_contained()`.

Hence this patch converts these assertions to runtime checks to
ensure that we check the arguments without assuming sanity at runtime.

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: I6433d65250f42787092a63a8a6e50d7fcb425dd2